Journal of Horticulture
Open Access

Regulated Dry Season Irrigation Effects on Tree Water Use, Root Zone Moisture Dynamics and Yield of Cacao in a Rainforest Zone of Nigeria

Agele Samuel Ohikhena^{* *}Correspondence: Agele Samuel Ohikhena, Department of Crop, Soil and Pest Management, Federal University of Technology, Akure, Nigeria, Tel: 8035784761, Email:

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Introduction

Cocoa (Theobroma cacao L.) is an important perennial fruit tree with an estimated annual world production of 3.2 million tonnes. Within the cocoa-growing belt of West Africa, sale of cocoa beans is a major foreign exchange earner, the cocoa sector employs millions of smallholder farmers (small farm sizes ranging from 0.5 hectare to 5.0 hectare (ha) and contributes about 70%-100% of their annual household incomes. In Nigeria, the main cocoa-producing areas are concentrated in the rainforest of the southern part of the country where an estimated 1.45 million hectares is cultivated. The productivity is 250 kg/ha, a yield level that is lower than those from Cote d’Ivoire and Indonesia (bean yields ranging from 600 kg/ha to 1000 kg/ha respectively). In the smallholder cocoa farms of West Africa, farm sizes are small ranging from 0.5 hectare to 5.0 hectare using low external inputs. The perennial fruit tree species of the rainforest of Nigeria: Cacao (Theobroma cacao), coffee (Coffea spp.) and kola (Kola spp.) are characterized by deciduous growth habit but are cultivated under rain fed conditions.

Global warming and drought and other climate-related disasters are tied to the changing climate, extreme and variability of the weather. The frequency and severity of drought event including (1.5°C to 2°C) warming are expected to increase in the near future as result of the decrease of regional precipitation and the increase in evapotranspiration driven by global warming. Among natural hazards, drought ranks first in terms of the number of people directly affected. The changing climatic events has implications for agriculture food security, economies, and welfare of the society and ecosystems. Projections of climate change have pointed to an increase in mean temperature (2°C by 2080°C) and potential/evapotranspiration, decrease in precipitation and crop (actual) evapotranspiration under future scenarios and uncertainties. This implies future yield decreases which can be associated with enhanced heat and water stress under future climate conditions. Thus, climate Change (temperature and rainfall) scenarios for the rainforest of Nigeria which have been variously constructed using climate models have indicated variabilities in rainfall pattern (amount, distribution, onset and cessation dates) and elevated maximum and minimum temperatures. These projected climatic changes will exacerbate soil moisture and thermal stresses during the dry season with implications for crop performance.

Cocoa is cultivated as a rain fed crop, and it is highly sensitive to soil and weather conditions of low rainfall, soil and air moisture deficit and temperature stresses. The changing growing environmental conditions (marginal soils and extreme weather events) impose constraints on cacao growth and productivity. In order to alleviate the constraints imposed by changing growing environmental conditions (marginal soils and extreme weather events) on cacao productivity, it is imperative to develop climatic-stress adaptive strategies for the fruit tree-based agroforestry systems of the rainforest tropics in the wake of changing climate/weather conditions (climate change and weather variabilities).

The FAO penman-monteith equation, is accepted worldwide as the standard method for estimating reference Evapotranspiration (ET_o). The reference Evapotranspiration (ET_o) is a measure of the evaporative demand of a given environment and thus crop consumptive water use which is the sum of evaporation from soil and plant transpiration from the field. The procedures to calculate ET_o from radiation, wind, humidity and temperature data are presented in the FAO paper no. 56. The standard procedure for estimating evapotranspiration is documented in the FAO I and D no. 56, where a list of K_c values for each crop and developmental stage is provided. This K_c approach has been used to obtain reference Evapotranspiration (ET_o) and crop consumptive water use (ET_c) for arables, trees and vines. The ratio between ET and ET_o, is defined as a crop coefficient (K_c). Thus, if K_c is known, the ET_c is calculated as:

ET_c= K_c^*ET_c          (1)

The FAO-56 dual crop coefficient approach also describes the relationship between crop Evapotranspiration (ET_c) and reference Evapotranspiration (ET_o) by separating the single K_c into the basal crop (K_cb) and soil water Evaporation (K_e) coefficients. In the FAO-56 single crop coefficient approach, the effect of both crop transpiration and soil evaporation are integrated into a single crop coefficient (K_c). However, crop Evapotranspiration (ET_c) estimation is more accurate by dual crop coefficient approach than the single crop coefficient approach, the dual crop coefficient approach uses more parameters and take soil practices and crop characteristics into consideration. In the dual approach a daily basal crop coefficient (K_cb), representing primarily the plant transpiration, and a daily soil evaporation coefficient (K_e) are considered separately according to the equation:

K_c= K_cb+K_e            (2)

ET_c=(K_cb+K_e)ET_o   (3)

K_cb is a transpiration coefficient and K_e is an evaporation coefficient. The K_cb and K_e are the basal crop and the soil evaporation coefficients. For the dual K_c method, K_s applies only to Transpiration (T_r) and provides actual Transpiration (T_a). The values of K_s are obtained using its relationship with measured water stress indicators. The FAO procedure for estimating crop consumptive use requirements provides a list of K_c values for each crop and developmental stage. In the K_c approach both crop transpiration and soil evaporation are timely averaged into a single coefficient (K_c) commonly used to obtain the ET_c for various crops. The FAO no. 56 publication offers the option of differentiating E from T_r by using a dual crop coefficient approach. The estimates of crop water requirements (ET_c) are derivable from the product of potential evapotranspiration of a reference crop (ET_o) using a crop factor (k_c). Based on these relations:

ET_c= ET_o^*k_c (4)

The crop coefficient k_c is estimated as:

k_c=ET_o/ET_c (5)

The crop coefficient (K_c) is based on a theoretical understanding of the processes of transpiration and evaporation from a tall crop, and assumes full crop cover or frequent wetting of the soil surface. Allen, et al., suggested a K_c value of 1.0-1.05 for a cocoa crop with a complete canopy.

Information is inadequate on the actual water use (ET_c) of cocoa on the field. Estimated values of ET_c ranging from 3 mm/day to 6 mm/day during rains and less than 2 mm/day in the dry season have been reported for cocoa. Field data (based on the sap flow method) suggest ET_c rates of less than 2 mm/day for cocoa crop with a complete canopy, this appear to be low compared with potential ET_o estimate of 3 mm d^-1-5 mm d^-1 using penman equation. In a simulated El Niˇno drought experiment reported by Moser, et al., in Indonesia, there were no significant differences in cocoa leaf, stem and branch wood, or fine root biomass production between the rain fed control treatment and the one in which rain through-fall was reduced by 70%-80% (for dry soil profile to permanent wilting point during the year). The combined average rate of water use by both cocoa and Gliricidia sepium (measured using heat dissipation sap flux sensors) was 1.3 mm. d^-1 in the protected plots reduced rain through-fall (rainfall reduction between 70%-80%) and 1.5 mm d^-1 in the control (70% of which was from the cocoa trees). These crop water use values were described as low. Over a consecutive 18 day period, transpiration average was equivalent to 1.31 mm d^-1, (about 10 liters per tree per day) compared with a penman potential ET_o estimate of 3 mm d^-1-5 mm d^-1 which equates to a crop factor (K_c) of about 0.3. The reports of cocoa water use and yield production when grown as mixed crop with coconut and once a week irrigation during November-December, once every six days during January-March and once in four to five days during April-May with 175 l water tree^-1. Maximum yields were obtained when cocoa was drip irrigated with 20 l tree^-1 day^-1 and the total irrigation amount was 175 l per tree. Assuming a planting density of 1600 trees ha^-1 (2.5 m × 2.5 m) these figures equate to water use values of 5.6 mm d^-1-7.0 mm d^-1 or 3.9 mm d^-1-4.8 mm d^-1, and 2.2 mm d^-1 or 3.2 mm d^-1 at 1100 trees ha^-1 (3 m × 3 m) under drip irrigation. No estimates of the yield benefits are given or the total quantity of water to be applied over a season. Based on field trials by Diczbalis, et al., the annual irrigation requirement was estimated as 470 mm, with peak weekly requirements of about 200 l tree^-1 (1250 trees ha^-1) while dry bean yields of between 1.5 t ha^-1 and 2.7 t ha^-1 was achieved from young trees.

The annual total rainfall in the cocoa growing regions of Nigeria is about 1500 mm (less than 2000 mm). The rainfall distribution pattern is bi-modal from April to July and September to November. There is a short dry period from July to August during which the relative humidity is still high with over cast weather conditions. There is a main dry season from November to February-March. The four to six months of dry weather results in soil water deficit and since irrigation is not part of the farming system, causing seedling mortality. In bearing plants, the existence of the short dry season during main crop pod filling can affect bean size if it is sufficiently severe. In adult plantings, water deficits result in lower yields and an increase in the level of mirid (capsid) damage. In the rainforest cocoa growing belt of West Africa, fruit trees in plantations (cacao, kola, coffee, citrus species and oil palm) are seldom irrigated especially during the terminal drought situation of the dry season. Few studies had addressed the responses of cacao to dry season irrigation especially, the effects of irrigation on root zone moisture dynamics, tree water use, growth and yield in the premise of unfavourable weather constituted by soil moisture deficit and high temperature stresses of the dry season.

Given the changing environment regimes (soil and weather/climate) and increasing worldwide demand for cocoa, it is important to develop sustainable production systems based on sound agronomic practices such as irrigation to ameliorate the extreme weather conditions (hydrothermal stresses), improve its productivity and extend frontiers of its production to marginal weather and soil conditions. Few studies had addressed these features in tropical trees and very little is known about cacao the responses of cacao to dry season irrigation in the premise of unfavourable weather constituted by soil moisture deficit and high temperature stresses. In addition, information is inadequate on water use of cocoa and dynamics of soil moisture extraction as affected by irrigation regimes. Experiments were designed to examine the effects of regulated dry season irrigation on root zone moisture dynamics tree water use, and bean yield of cacao in a rainforest zone of Nigeria.

Materials and Methods

Experimental site and conditions

An experiment was conducted on the research and experiment station of the department of crop, soil and pest management, federal university of technology Akure, Nigeria. Akure is located in the rainforest zone of South West Nigeria on latitude: 7º18I N, longitude: 5º81 E and 350 m abs. Five to six years old fruiting cacao trees which had been previously irrigated during dry season from seedling establishment till date were used.

The cocoa-growing rainforest belt of Southern Nigeria, is characterized by wet and dry season transition, and the seasons have variable weather conditions. The annual rainfall range from 1500 mm to over 2000 mm distributed in a bimodal pattern within seven to eight months duration and 3 months to 4 months of dry season. The dry season is a terminal drought situation characterized by inadequate rainfall, soil moisture, high vapour pressure deficit and temperatures stresses and very clear sky (high intensity of solar radiation). In the rainforest cocoa growing belt of West Africa, fruit trees in plantations (cacao, kola, coffee, citrus species and oil palm) are seldom irrigated especially during the terminal drought situation of the dry season.

Soil characteristics and moisture determination

The soil of the site of experiment is sandy-clay-loam with relatively high water holding capacity. Available soil water in the upper 0.60 m of the soil depth is 187 mm. the percent and volumetric soil water contents at field capacity and permanent wilting point are 21% and 10% respectively. Mean bulk density was 1.25 g cm^-3.

Soil samples were taken using soil Auger for water content measurement within the top soil layer (0 cm-30 cm) by gravimetric method. Core samples were taken for bulk density and porosity measurement. Soil moisture content would attain field capacity in two days since the soil is sandy clay to silty clay loam. The samples were taken two days after and just before the next irrigation. The difference in moisture content between the two sampling periods was taken to be the moisture used. That is, the evapotranspiration by the crop for that period. Since it was assumed that drainage was negligible (no drainage), the moisture change was principally attributed to evapotranspiration. Soil Moisture Depletion (SWD) was obtained from the differences in soil moisture contents (changes in soil moisture contents: (S) measured between two measurement period. Soil moisture contents were determined weekly at 20 cm depths from soil samples taken with augers and core samplers.

Irrigation strategies

Cacao trees were drip-irrigated based on levels of cumulative pan evaporation. Irrigation treatments were based on the restoration of cumulative Pan Evaporation (E_Pan) using variable Pan Coefficients (K_cp) of 100%, 70% and 50%. The pan coefficients (100%, 70% and 50% K_cp; the relative water deficit of 0, 0.3 and 0.5) indicated zero, high and low water stress conditions and respectively. Irrigation amount was calculated using pan evaporation and pan coefficients (K_cp1: 1.0; K_cp2: 0.7, and K_cp3: 0.5) according to Doorenbos and Pruitt and Allen, et al. as:

I_r=A^*E_pan^*K_cp (6)

Where:

I_r=Amount of applied Irrigation water (mm);

A=Plot area, E_Pan is the cumulative evaporation at irrigation interval (mm) and K_cp=Plant-pan coefficient.

Irrigation treatments were coded as E_Pan^*100 K_cp (IrT1), E_Pan^*70% K_cp (IrT2) and E_Pan^*50% K_cp (IrT3) while irrigation was fixed at 5 days-interval for the three irrigation treatments. Irrigation treatment IrT3 had the maximum water deficit which was used to determine stressed baseline while IrT1 suggest adequate irrigation to meet full crop water requirements (the non-crop water stress baseline). Irrigation water was applied using gravity-drip irrigation system at 4.8 l/tree/day, 6.8 l/tree/day and 9.6 l/tree/day at each irrigation via point source emitters of 2 l/h discharge rate which were installed on laterals per row of cacao tree spaced at 3 m × 3 m. One drip lateral served each plant row and an inflow meter was installed at the control unit to measure total flow distributed to all replications in each treatment. Irrigation buckets were suspended on 3.5 m high tank stands to provide the required hydraulic heads.

Total amount (volume) of irrigation water applied per treatment was calculated using equation:

V=P^*A^*E_Pan^*K_cp (7)

Where:

V=Volume of irrigation water (L);

P=Wetting percentage (taken as 100 % for row crops);

A=Plot area (m²);

E_pan=Amount of cumulative evaporation for the irrigation interval (5 days) and

K_cp=Pan coefficients (1.0, 0.7 and 0.5).

This corresponded to 7.14 mm (1.93 l/day), 10.7 mm (2.90 l/day), 14.28 mm (3.86 l/day) for the respective 0.5 K_cp, 0.7 K_cp and 1.0 K_cp. In order to attain good plant stand, a pre-treatment total of 135 mm of irrigation water was applied equally to all treatment plots in several applications, this replenished soil water in the 0.60 m profile depth to field capacity across treatments. Following the pre-treatments of 4.82 l/day for 5 days, differential irrigation treatments commenced on 13^th December, 2017 and was terminated May 9^th, 2018. The amount of water applied per irrigation and seasonal irrigation amount varied from a maximum of 4.82 l/day and 127500 mm (D_I1 level) to a minimum of 1.93 l/day and 20400 mm (D_I4 level). Irrigations continued until one week before the final harvest. Actual crop Evapotranspiration (ET_c) of cacao under the irrigation amounts was calculated with the water balance equation (equation 1).

ET+I+P+ΔS-D_p-R_f (8)

Where:

ET=Actual crop Evapotranspiration (mm);

I=Amount of Irrigation water applied (mm);

P=Precipitation (mm);

ΔSW=Changes in the soil water content (mm);

D_p=Deep Percolation (mm);

R_f=Amount of Runoff (mm).

Since the amount of irrigation water was controlled, deep percolation and run off were assumed to be negligible. Daily crop evapotranspiration was estimated using the pan evaporation data, pan factor and crop coefficient. Data for Pan Evaporation (E_Pan) used for the experiment were obtained from measurements with class-a pan (121 cm in diameter and 25.5 cm in depth) from the meteorological station, department of meteorology and climate science, futa) located near the plots.

Deep percolation was considered as zero because there was no high underground water problem in the area. If available water in the root zone (0 cm-90 cm) and total applied water amount by irrigation were above the field capacity, it would be assumed that water amount above field capacity leaked into the deeper soil zones and was called deep percolation (D_p: Available total water amount at 0 cm-90 cm soil depth before irrigation+applied irrigation water field capacity). Total Water Requirement (WR) was determined using the relation:

WR=A × B × C × D × E (9)

Where;

WR=Water Requirement (l per day/plant);

A=Open pan evaporation (mm/day);

B=Pan factor (1.0, 0.7 and 0.5);

C=Spacing of plant (m²);

D=Crop factor (factor depends on plant growth, value for fully grown cacao=1.13 but for cacao in the early fruiting stage, 0.83 was adopted).

Water Requirements (WR) were 9.63 l/plant/day, 6.75 l/plant/day and 4.8 l/plant/day for the respective IrT1, IrT2 and IrT3 irrigation treatments.

Irrigation water requirement is determined using average season wise pan evaporation data for the area. The Total Water Requirement (TWR) of the farm plot was obtained using the relation. Therefore, the Total Water Requirement (TWR) of the farm plot is:

TWR=WR × no. of plants (10)

Maximum Allowable Deficit (MAD) for cacao (50% of available water storage capacity of the soil (AWC) Gross Irrigation Requirement (GIR) of an orchard or vineyard, the computed ET_c, which is considered as the Net Irrigation Requirement (NIR), should be divided by the application efficiency (AE).

GIR=NWR/AE (11)

Yield and crop water use were deployed to evaluate appropriate the efficiencies of irrigation management practices among the different irrigation strategies adopted.

Orchard water use efficiencies

Water productivity (Irrigation Water Use Efficiency (IWUE) and crop water use efficiency (WUE) was determined based on the methods of Sezen, et al., and Agele, et al., as:

IWUE=Biomass weights (Y)/Total irrigation water applied (I_r) (12)

WUE (crop)=Biomass weight (Y)/Cumulstive seasonal ET_c (13)

Where:

IWUE=Irrigation Water Use Efficiency (t.ha^-1 mm),

EY=Economical Yield (t.ha^-1), Ir=Amount of applied Irrigation water (mm).

Cacao water requirement was determined using FAO-56 single and dual crop coefficient models approach. The aim was to analyze the capacity of the FAO-56 single and dual crop coefficient models to assess cacao evapotranspiration and water requirements (estimating adequacy of irrigation amount for cacao). The FAO-56 dual crop coefficient approach describes the relationship between crop Evapotranspiration (ET_c) and reference Evapotranspiration (ET_o) by separating the single K_c into the basal crop ( K_cb) and soil water evaporation (K_e) coefficients, while in the FAO-56 single crop coefficient approach, the effect of both crop transpiration and soil evaporation are integrated into a single crop coefficient (K_c). Cacao orchard irrigation was scheduled from transpiration and evaporation coefficients (K_cb, K_e). FAO I and D No. 56 publication offers opportunity for differentiating E from T_r by using a dual crop coefficient approach, according to the equation:

ET_c=(K_cb+K_e)ET_o (14)

Where k_cb is a transpiration coefficient and K_e is soil evaporation coefficient.

K_cb is basal crop coefficient (K_cb=ET_c/ET_o).

K_c=K_cb+K_e (15)

And then,

ET_c=(k_cb+k_e)ET_o (16)

Size of cacao canopies

Tree canopies may be characterized using two parameters: canopy volume (m³ of tree volume/m² of ground surface) and leaf area density (m² of leaf area/m³ of tree volume). Tree canopy can be measured with a measuring rod once the tree shape has been approximated as a sphere, an ellipsoid, or a truncated inverted cone. As an alternative to the measurements or calculations of the radiation actually intercepted by the tree, a simple parameter that is easy to determine is the degree of ground cover. The ground cover (normally expressed in percentage) is obtained by measuring the shaded area outlined from the horizontal projection of the tree canopy.

A=[πd4²/4] (m²) (17)

Where: d₄=diameter of shaded area by cacao canopy (2 m);

A=Per cent ground cover by cacao canopy;

Tree spacing=3 m × 3 m (9 m²);

d₁=(areal canopy area);

d₂=(height bt d₁ and d₃);

d₃=(projection of canopy area on ground, d₁> d₃).

Canopy volume=1/3πd₂[(d₁x²/4)+(d₃x²/4)+d₁d₃)] (18)

Soil surface evaporation as affected by irrigation

Soil evaporation from surface not wetted by emitters (Ed_z: The rest of the soil surface outside the emitter wetting pattern) and surface evaporation from the soil wetted by the emitters (emitter wetted zone (EW_z) were estimated.

The equations derived from research on an olive orchard ET by Orgaz et al., were adapted for estimating the average monthly value of EW_z and WD_z

Evaporation (ED_z) from soil not wetted by emitters (ED_z)

ED_z=K_s, e ET_o (mm/day) (19)

Where:

G=Ground cover fraction of tree canopy, is monthly rainfall amount;

W_z=Fraction of soil surface wetted by drip emitters (ET_o=reference ET);

EW_z=0.6 ET_o for dense crop cover/plant density;

EW_z=Soil surface kept wet by emitters, and Cover crop coefficient varies from 0.25 to 0.8;

Evaporation (ED_z) from soil not wetted by emitters (ED_z).

ED_z=K_s, e ET_o (mm/day) (20)

Cover crops/weed cover transpiration (T_r cc).

Weed cover up to 2 m in a 3 m row spacing.

Transpiration (T_r) of cacao orchard

Cacao is deciduous (partly evergreen in some cases), its crop coefficient (K_c) and T_r were determined using the methods of Orgaz, et al.

K_c, T_r=(QdF₁) F₂ (21)

Qd=1-e-k_ext V_u (22)

Where:

K_ext=Radiation extinction coefficient;

K_ext=0.52+0.00079d_p-0.76e-1.25DAF (23)

DAF=2-0.53(V_u-0.5) (24)

Where:

DAF must be>2; V_u=V_o(d_p/10000).

V_o=1/6 × D²H;

E=Exponent=2.718;

H=Height of canopy, m

D=Average canopy diameter, m

V_o=canopy volume; m³/tree

V_u is canopy volume as amount on ground cover, m³/m²

DAF is leaf area density;

d_p is tree density; number/ha

F_i=0.07 for tree density greater than 300 trees/ha

F₂ is monthly coefficient of T_r which is about 0.7 to 1.0 from wet to dry seasons.

Qd=1-e-k_extV_u

K_c, T_r=(QdF₁)F₂ (25)

ET_c+ET_oK_cK_r,t (26)

K_c, t is empirical coefficient relating the ET of an orchard of incomplete cover to a mature orchard of full canopy cover. In addition, K_r, t relates to horizontal projection of tree shade/canopy, and Kr,t is about 0% to 70% of G.

Cacao orchard Transpiration (Tr) was determined as:

T_r=K_c, T_r ET_o (27)

Where K_c, T_r is transpiration coefficient which varies bt 0.75 to 1.0 seasonally until leaf senescence onset for a fully wetted orchards (sufficient soil moisture situation) (K_c, T_r decrease at senescence and recovers at the onset of rainfall.

Results and Discussion

Weather conditions during period of study

The late (minor) rainy season (mid-August to December) is characterized by high cloud overcast (overcast sky), low air temperatures and higher relative humidity compared with the major rainy season (April to mid-August) and the dry season (Figure 1). On the average, the rainy season had higher mean relative humidity averaged (71%) and lower air temperatures (32.8°C) compared with the dry season (December to March). Also, higher air temperature and VPD and lower relative humidity were found for the unshaded open sun cacao compared with the shaded plants.

Figure 1: Weather variables during period of experiment (December to May) showing ambient temperature, rainfall, humidity, vapour pressure deficit and pan evaporation.

A low pressure gravity-drip system was deployed to deliver water to cacao root zone which alleviated moisture stress during the dry season. Across sampling dates, different amounts of irrigation water were applied based on cumulative Pan Evaporation (E_Pan)* pan coefficients for the respective irrigation treatments IrT1 (E_Pan.k_c:1.0), IrT2 (E_Pan.K_c: 0.7) and IrT3 (E_Pan.K_c: 0.5). Irrigation amounts on monthly averages were 1009.88 mm, 706.91 mm and 504.94 mm while seasonal totals were 2116.5 mm, 8482.95 mm and 6059.25 mm for IrT1; IrT2 and IrT3 treatments. Well irrigated treatment (IrT1, E_Pan.k_c: 1.0) had highest amount delivered and lowest by IrT3 (E_Pan.k_c: 0.5) (Figure 2). The deficit irrigations (IrT2 and IrT3) delivered 79% and 68% respectively water to cacao root zone compared with the well irrigated condition. Peak and significantly higher values of irrigation amount were delivered at DOY 45, 60, 75 and 90, periods which coincided with highest E_Pan values (>5 mm/day). The irrigation treatments (IrT1, IrT2 and IrT3) affected soil moisture contents within cacao root zone. Soil moisture contents adequately reflected the irrigation water delivered across measurement dates. Highest soil moisture contents were obtained for well irrigated (IrT1) and lowest for deficit irrigation (IrT3) treatment. For the respective deficit irrigation treatments (IrT2 and IrT3: 0.7 and 0.5 E_Pan coefficients), average soil moisture contents were 61%, 48% and 42% for IrT1, IrT2 and IrT3 irrigation treatments (Figure 3). Highest soil moisture contents and crop evapotranspiration (ET_c) were obtained from well irrigated plots (IrT1: E_Pan*k_cp (1.0 (9.6 l/tree/day) followed by IrT2 (E_Pan*k_cp (0.7) (6.8 l/tree/ day) and lowest for IrT3 E_Pan*k_cp (0.5) (4.8 l/tree/day). The deficit irrigation treatments (IrT2 and IrT3) had lower soil moisture contents (14.7 and 11.8 %) which equated to 30% and 50% water savings [1].

Figure 2: Time course of irrigation water applied during the perido of experiment (December to May). The three irrigation treatments were coded as: IrT1 ( E_Pan*1.0 k_cp), IrT2 (E_Pan*0.7 k_cp) and IrT3 (E_Pan*0.5 k_cp). E_Pan is Pan Evaporation and k_cp is Pan.

Figure 3: Dynamics of soil moisture contents as affected by irrigation treatments. The three irrigation treatments were coded as: IrT1 ( E_Pan*1.0 k_cp), IrT2 (E_Pan*0.7 k_cp) and IrT3 (E_Pan*0).

Declines in soil moisture contents were obtained from DOY 345 to DOY 60, followed by increasing trends in soil moisture from DOY 75 till end of measurement (DOY 150). Declining trends in the values of soil moisture contents may be attributed to the increasing intensities in climatic demand (high vapour pressure deficits). Unfavourable weather of high temperatures and soil evaporation and low atmospheric humidity would enhance soil moisture depletion thus the low soil moisture status. Increases in moisture were observed from DOY 75 till end of measurement (DOY 150) can be attributed to rainfall received following its commencement (Mid-March). In general, the observed trends in the status of root zone moisture is attributable to the prevailing weather conditions denoted by increasing intensities of climatic demand (vpd) and temperatures during periods (DOY 345 to 60) of experiment.

Cycle of soil water before and after irrigation

Soil water contents were measured using soil samples within the 0 cm-20 cm soil profile depth before and one day after each irrigation. Soil moisture contents across measurement days ranged between wilting point (140 mm) before irrigation and field capacity (260 mm) after irrigation (data not shown). For the low and high water stress conditions (IrT2 and IrT3), soil moisture was often close to wilting point before each irrigation [2]. For the deficit irrigation treatments (IrT2 and IrT3:0.7 and 0.5 pan coefficients), available water fell below 50% more often than not during the period of study. Because much more water was applied under high pan coefficients (K_cp 1.0), soil moisture contents of well-watered treatments (IrT1) was higher compared with deficit irrigation (IrT2:0.7 K_cp and IrT3:0.5 K_cp) treatments. The well-watered treatment (IrT1), most times, maintained soil moisture within field capacity range.

In general, based on the values of soil moisture, the stored water within crop root zone profile was used up between irrigation cycles. This is attributable to the intensities of climatic stress (high temperatures and vapour pressure deficits) which presumably enhanced soil evaporation and the rapid depletion of water stored in the soil profile. Soil moisture content immediately following irrigation gradually decreased towards next irrigation event, this situation confirms the inability of soil moisture reserve to satisfy cacao water demand during the dry season which was consistent with earlier reports of Famuwagun, et al., and Charles, et al.

Soil moisture depletions over two measurement days were deployed to determine cacao water use (ET_c). Cacao water use (ET_c) differed across measurement dates and irrigation treatments (Figure 4). Average cacao evapotranspiration (ET_c) were 139 mm/day, 97 mm/day and 63 mm/day for the respective IrT1 (IrT1 (K_c:1.0), IrT2 (K_c 0.7) and IrT3 (K_c 0.5). Cacao evapotranspiration (ET_c) for the deficit irrigation treatments (IrT2: 0.7 and IrT3:0.5 E pan coefficients) were averagely 45% and 70 % less compared to values to adequate irrigation (IrT1) treatment which signified soil moisture deficit stress [3]. Peak ET_c values were obtained at DOY 45, 60 and 75 possibly due to high E_pan (>5 mm/day), lowest for DOY 120 and 135 with increases afterwards. The increases in cacao water use (ET_c) from DOY 135 afterwards are attributable the commencement of rainfall and associated replenishment of soil moisture, lowering of temperatures (air and soil) and high atmospheric humidity (declining atmospheric demand). The well-watered treatment (IrT1) had highest ET_c and the more deficit irrigation (IrT3) had least cacao water use. The mean ET_c across measurement dates were 5.07 mm/day, 3.55 mm/day and 2.63 mm/day for IrT1, IrT2 and IrT3 irrigation treatments for the period of experiment (December to May).

Figure 4: Cacao water use (Evapotranspiration; ET_c), Time course of cacao water use (Evapotranspiration; ET_c). The three irrigation treatments were coded as: IrT1 ( E_Pan*1.0 k_cp), IrT2 (E_Pan*0.7 k_cp) and IrT3 (E_Pan*0.5 k_cp). E Pan is Pan Evaporation.

In addition to single crop coefficient (k_c=1.31), cacao water requirement (ET_c) was also computed using the dual (krt:1.04) crop coefficient. Means of cacao water use for dual crop coefficient across measurement dates were 5.2 mm/day, 3.7 mm/day and 2.8 mm/day for IrT1, IrT2 and IrT3 irrigation treatments for the period of experiment (December to May). The time course of cacao water use estimated using both the single and dual crop coefficients are presented in Figure 5. Results showed similar trends in cacao ET_c for both methods and irrigation treatments while values were higher for the dual coefficient compared with the single k_c (Figure 5). The decreasing order of ET_c for single k_c and dual k_c were IrT1>IrT2>IrT3. Crop Evapotranspiration (ET_c) increased with increases in the volume of irrigation water applied, this modified values of ET_c obtained for both the single and dual k_c approaches [4].

Figure 5: Time course of cacao water use (Evapotranspiration; ET_c). The three irrigation treatments were coded as: IrT1 ( E_Pan*1.0 k_cp), IrT2 (E_Pan*0.7 k_cp) and IrT3 (E_Pan).

The FAO-56 dual crop coefficient approach which describes the relationship between crop Evapotranspiration (ET_c) and reference Evapotranspiration (ET_o), separates the crop coefficient (K_c) into the basal crop (K_cb) and soil water evaporation (K_e) coefficients. In the single crop coefficient approach, the effect of both crop transpiration and soil evaporation are integrated into a single crop coefficient (K_c) while in the dual coefficient approach, a daily basal crop coefficient (representing plant transpiration: K_cb) and daily soil evaporation coefficient (K_e) in the form of K_c=K_cb+K_e). However, crop Evapotranspiration (ET_c) estimation is more accurate by dual crop coefficient approach than the single crop coefficient approach, the dual crop coefficient approach uses more parameters and take soil management practices and crop characteristics into consideration (Figure 6).

Figure 6: Ratio of cacao water use (ET_c) to pan Evaporation (E_Pan) as affected by irrigation regime. The irrigation treatments were coded as: IrT1 (E_Pan*1.0 k_cp), IrT2 (E_Pan*0.7 k_cp) and Ir.

The magnitudes of cacao ET_c (single and dual crop coefficients obtained from the respective irrigation treatments followed from the irrigation water delivered (Figure 7). The irrigation regimes affected soil moisture contents and thus, its availability to meet crop water use. The values of cacao water use obtained from the respective irrigation treatments would have followed from the irrigation water delivered. Irrigation under well-watered treatment increased tree water use and soil moisture status compared with deficit irrigation treatments (IrT2 and IRT3) which is consistent with reports on citrus by Yang, et al. The magnitude of cacao ET_c obtained in this study are within the range of those reported in literatures [5]. Cacao water use (ET_c) values ranging from 3 mm/day to 5 mm/day during rains and less than 2 mm/day in the dry season has been reported under irrigation regime of 10 litre/tree/day, Kohler, et al., obtained cacao ET_c of 2 mm/day and Moset, et al., obtained 1.3 mm/day-1.5 mm/day in Indonesia. Cacao transpiration average of 1.31 mm d^-1 (about 10 liters per tree per day) and ET_c computed with a Penman potential ETo of 3 mm d^-1-5 mm d^-1 have been reported. This value equates to crop factor (K_c) of about 0.3. Field data (based on the sap flow method) suggest ET_c rates of less than 2 mm/day for cocoa crop with a complete canopy, this appear to be low compared with potential ET_o estimate of 3 mm d^-1-5 mm d^-1 using penman equation (Figure 8).

Figure 7: Ratio of cacao water use (ET_c) to irrigation water applied, Ratio of cacao water use (ET_c) to irrigation water applied. The irrigation treatments were coded as: IrT1 (E_Pan *1.0 k_cp), IrT2 (E_Pan *0.7 k_cp) and IrT3 (E_Pan*0.5 k_cp).

Figure 8: Time course of cacao water use (ET_c) by single and dual crop coefficients (k_c): k_c (1.13), k_r t (1.04) for well irrigation treatment (IrT1; E_Pan*100 k_cp).

The ET_c/E_Pan ratio denotes the proportion of climatic water demand satisfiable by crop water use (ET_c). Among the irrigation treatments. The proportions of Pan Evaporation (E_Pan) to cacao water use (ET_c) denoted as ET_c/E_Pan ratio, differed across measurement dates and irrigation treatments. The means of ET_c/E_Pan ratios across measurement dates were 1.016, 0.714 and 0.492 for the respective IrT1, IrT2 and IrT3 treatments (Figure 9) [6,7]. ET_c/E_Pan curves were similar but the ratios ranged from 1.16 to 0.50 which indicated that both climatic demand (E_Pan) and cacao water consumption (ET_c) were high during the dry season at the site of study.

Although a weak relationship, linear regression equation was fitted to the ET_c/E_Pan trends as: Y=0.011 × +0.94, R²=0.32). The ratio of water use (ET_c) to irrigation water applied denotes the proportion of irrigation water applied used for crop evapotranspiration. Trends of ET_c to irrigation were similar among irrigation treatments and measurement dates but values differed among irrigation treatments (Figure 10). The mean values were 1.016, 0.714 and 0.492 for the respective IrT1, IrT2 and IrT3 treatments. Generally, the ratios ranged from 1.13 to 0.27, 0.79 to 0.19 and 0.57 to 0.14 which indicated differences in the ability of irrigation water applied to satisfy climatic demand (E_Pan) driven trends of cacao water consumption (ET_c) (Figure 11). When soil water is readily available to a crop, the rate of water evaporation from an evaporation pan is proportional to the rate of crop water use. Doorenbos and Kassam found positive linear and significant logarithmic correlation (P<0.01) between ET_c and EP_a. While Smajstrla, et al. obtained significant logarithmic correlation (P<0.01) between ET_c and E_Pan. These reports confirmed the established close relationship between plant water consumption and pan evaporation.

Figure 9: Time course of cacao water use (ET_c by single and dual k_c: k_c (1.13), kr t (1.04) IrT1:1.0 E_Pan and E_Pan (IrT1; E_Pan*0.7 k_cp).

Figure 10: Time course of cacao water use (ET_c by single and dual k_c: k_c (1.13), kr t (1.04) IrT1:1.0 E_Pan and E_Pan (IrT1; E_Pan*.0.5 k_cp).

Figure 11: Time course of monthly evaporation from witted (ED_z) and non-witted (EW_z) within cacao field. Monthly evaporation from witted (ED_z) and non-witted (EW_z) within cacao field during the period of experiment (December to May).

Soil evaporation from cacao orchard

The ET_c from an orchard is more complex. In addition to tree T_r, there could be T_r losses from cover crop or from weeds, and there are E losses from the soil. Under irrigation conditions, there are two E components that may differ in their rates: One is the E from the soil areas wetted by the emitters, and the other is the E from the rest of the soil surface which is only wetted by rainfall. Soil evaporation was respectively estimated for the wetted zone (Ed_z) and the non-wetted zone (Ew_z) during the period of experiment. The mean values for soil evaporation for the wetted (Ed_z) were 5.65 mm/month, 2.82 mm/month and 0.19 mm/month for the respective IrT1, IrT2 and IrT3 treatments. The seasonal totals soil evaporation for the wetted (Ed_z) and the non-wetted (Ew_z) zones were 234.29 mm and 33.94 mm respectively. Based on the cumulative seasonal totals, soil evaporation for the wetted zone (Ed_z) was averagely 7 times compared with the non-wetted zone (Ew_z). Irrigation replenished soil moisture depletion while cacao canopy produced cover to soil and a more favourable microclimate around the canopy spread. This is appear to explain the magnitudes of soil evaporation from the wetted (Ed_z) and the non-wetted (Ew_z) zones within cacao field. Studies also indicated that the conditions at the soil surface due to (i) the percentage of soil surface wetted via irrigation, (ii) the irrigation intervals and (iii) the soil exposure to light determine the dynamics of T_r and E_s in orchards [8].

In this study, drip irrigation was deployed to replenish moisture depletion from cacao root zone. There were spatial variations in the degree of wetting within the orchard; some areas are frequently wetted by the emitters while the rest of the soil surface remains dry in the absence of rainfall. The drip lines were placed near the trees while the wetted areas are shaded by the cacao canopy. The effects of orchard canopy and drip irrigation appeared adequate to alleviate radiation-limited soil water evaporation (E) [9]. Measurements and models suggest that E from the soil surface in orchards, which are wetted frequently (every 1-2 days) by emitters is equivalent to about 60 percent of the ET_o from the wet areas. As a first approximation, the quantification of E from the wetted spots in a drip-irrigated orchard can be made using a semi-empirical model of Bonachela of the relation:

E_s=0.6 ET_o (28)

Total pod and bean yields were highest for IrT1 (35.4 t.ha^-1 and 2.29 t.ha^-1) followed by IrT2 (22.1 t.ha^-1 and 1.37 t.ha^-1) and lowest (10.3 t ha^-1 and 1.03 t ha^-1) for IrT3. Bean yields decreased by 60% and 40% under IrT3 and IrT2 compared with IrT1 (Table 1). Deficit irrigations however produced 30% and 50% water savings compared to well-watered treatment (IrT1). Water productivity was affected by irrigation regimes. Water use efficiencies values ranged between 0.3 t/mm and 0.04 t/mm for Y/ET_c and 0.16 kg/mm to 0.19 kg/mm for Y/Irrigation respectively [10].

Table 1: Summary of measured soil and cacao variables as affected by irrigation regimes.

Irrigation treatments were: 585 IrT1: E_Pan*1.0 k_cp, IrT2: E_Pan*0.7 k_cp and IrT3: E_pan*0.5 k_cp. For the analysis of the data, the statistical 586 significance of irrigation treatment on the measured variables was indicated by the LSD. The significance of 587 differences between treatment means were explored at P<0.05. Irrigation requirement was based on cumulative class 588 A Pan evaporation and pan coefficients (k_cp).

The yields of pods and beans were significantly higher in IrT1 treatments compared with IrT2 while they were lowest significantly for IrT3. However, Carr and Charles, et al., had reported that fruit yield does not only depend on irrigation amount but a function of other management practices adopted and soil properties such as infiltration rate, and water holding capacity [11].

Irrigation effect was significant both on number and weight of pods and beans in cacao. Evaluation of irrigation amount and frequency should not only consider fruit yield and yield components, but also consider WUE. Other studies have reported irrigation effects on biomass, pod and bean yields of cacao. Dicbalis, et al., working in Australia examined effects of seasonal irrigation requirement of 470 mm and 200 l/tree for weekly irrigation and obtained resultant bean yields of 1.5 t/ha-2.7 t/ha. Based on field trials by Diczbalis, et al., the annual irrigation requirement was estimated as 470 mm, with peak weekly requirements of about 200 l tree^-1 (1250 trees ha^-1) while bean yields of between 1.5 t ha^-1 and 2.7 t ha^-1 were obtained as achieved from young trees.

Cocoa is cultivated as a rain fed crop but sensitive to weather extremes of low rainfall, soil moisture deficit and high temperature stresses had been variously reported in the literature [12]. Global warming (including 1.5°C to 2°C), drought and other climate-related disasters are tied to the changing climate, extreme and variability of the weather. These situations would drive decreases of regional precipitation and increase in evapotranspiration driven [13]. These conditions may enhance yield decreases associated with enhanced heat and water stress under present and future climate conditions. The climate models of the rainforest of Nigeria have been variously constructed. The results indicate that the projected climatic changes will exacerbate soil moisture and thermal stresses with implications for crop performance [14]. As precipitation becomes more variable and unpredictable in addition to the expected increased warming due to changing climate, development of water-saving management practices for sustainable agriculture now and in the future is envisaged [15]. Establishing the optimal irrigation scheduling is important in the development of water-saving practices for sustainable cacao production and climate stress alleviation in the wake of the hydrothermal (extreme heat and water deficits) stresses envisage under future climate.

The site of study in the rainforest zone of Nigeria is characterized by bi-modal rainfall pattern and the wet-dry season transition. The rainfall distribution pattern is bi-modal from April to July and September to November. The dry season which span December of a year to April of the other, is a terminal drought situation characterized by inadequate rainfall, soil moisture deficit, high vapour pressure deficit and temperatures and very clear sky (high intensity of solar radiation [16]. Such unfavorable weather condition will enhance hydrothermal stresses, evapotranspiration, leaf senescence, and branch and twig die-back and even tree mortality [17].

Conclusion

The variable irrigation based on variable pan coefficients (1.0, 0.7 and 0.5) affected dynamics of 469 root zone moisture, cacao water use (ET_c) and yields. Study established suitable pan coefficients for scheduling irrigation during the terminal drought situation of the dry season in a rainforest zone of Nigeria. Relative to adequate irrigation (IrT1), deficit irrigations (IrT2 and IrT3) produced lower soil moisture, water use (ET_c) pod and bean yields which were however accompanied by 25% to 44% increases in water use efficiencies and 30 and 50 % water savings. The water saving advantage of deficit irrigation strategies can be scaled up for adoption. The low pressure gravity drip irrigation system alleviated climate stress during the dry season and improved cacao performance. Findings will be of relevance to adaptation strategy in the premise of the changing climate, extreme and variability of weather while measured cacao water use (ET_c) will find use 478 for improving irrigation models for fruit trees.

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