Effects of regulated dry season irrigation on tree water use, root zone moisture dynamics and yield of cacao in a rainforest zone of Nigeria


 A field trial was conducted to investigate the effects of regulated dry season irrigation on tree water use, root zone moisture dynamics and yield of cacao in a rainforest zone of Nigeria. Following cessation of rainfall in November, irrigation commenced from December 2017 to May 2018. Irrigation amount was computed based on cumulative class A Pan evaporation. Irrigation treatments were coded as IrT1, IrT2 and IrT3, consisting of water application using EPan *Pan coefficients (Kcp) of 1.0; 0.70 and 0.50 (9.6, 6.8 and 4.8 l/tree/day). Irrigation water applied at 5-days interval was discharged via point source emitters (2.8 l/h discharge rate ) on drip lines laterally installed per row of trees. Irrigation requirements were on the average, 4.49, 3.14 and 2.44 mm, total water applied per irrigation events were 1009.88, 706.92 and 504.94 mm per plot ( 225 m2 ), total seasonal water applied were 33858, 23701 and 16929 mm, and soil moisture contents were 52, 45 and 28% for the respective IrT1, IrT2 and IrT3. Tree evapotranspiration (ETc) were 4.54, 3.19 and 2.32 mm/day while seasonal sums were 809, 566 and 404 mm while the ratio of ETc to EPan were 0.9, 0.69 and 0.53 for IrT1, IrT2 and IrT3. Tree water use efficiencies were 0.3 and 0.04 t/mm for Y/ETc and 0.16 to 0.19 kg/mm for Y/Irrigation respectively. Cacao pod and bean yields were 35.4, 22.1 and 10.3 t/ha and 2.29, 1.37 and 1.03 t/ha while yields decreased by 60 and 40% under IrT3 and IrT2 compared with IrT1. The study identified suitable Pan coefficients for scheduling irrigation during the dry season for cacao, full irrigation (EPan*1.0) applied at 9.6 l/tree/day will be needed to replenish soil water depletion to satisfy crop consumptive water use ( transpiration and soil evaporation components). The low pressure gravity-drip irrigation system alleviated climate stress during the dry season and improved cacao performance in a tropical rainforest environment.


Introduction
Cocoa (Theobroma cacao L.) is an important perennial fruit tree with an estimated annual world production of 3.2 million tonnes (FAO, 2012). 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 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 to 1000 kg/ha respectively). In the smallholder cocoa farms of West Africa, farm sizes are small ranging from 0.5 to 5.0 hectare using low external inputs. The perrenial 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 rainfed conditions (Opeke, 2006 Cocoa is cultivated as a rainfed crop, and it is highly sensitive to soil and weather conditions of low rainfall, soil and air moisture de cit and temperature stresses (Opeke, 2006, Zuidema et al., 2005, Charles et al., 2019. 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 (ETo). The reference evapotranspiration (ETo) 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 eld (Penman, 1948, Doorenbos and Pruit, 1977, Ventura et al., 1999, Allen et al., 2005. The procedures to calculate ETo 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&D No. 56, where a list of Kc values for each crop and developmental stage is provided. This Kc approach has been used to obtain reference evapotranspiration (ETo) and crop consumptive water use (ETc) for arables, trees and vines (Allen et al., 1998, Ferreira, 2017. The ratio between ET and ETo, is de ned as a crop coe cient (Kc). Thus, if Kc is known, the ETc is calculated as: The FAO-56 dual crop coe cient approach (Allen et al., 1998, Allen andPereira, 2009) also describes the relationship between crop evapotranspiration (ETc) and reference evapotranspiration (ET0) by separating the single Kc into the basal crop (Kcb) and soil water evaporation (Ke) coe cients. In the FAO-56 single crop coe cient approach, the effect of both crop transpiration and soil evaporation are integrated into a single crop coe cient (Kc). However, crop evapotranspiration (ETc) estimation is more accurate by dual crop coe cient approach than the single crop coe cient approach, the dual crop coe cient approach uses more parameters and take soil practices and crop characteristics into consideration (Monteith, 1973, Ventura et al., 1999, Zhang et al., 2013). In the dual approach a daily basal crop coe cient (Kcb), representing primarily the plant transpiration, and a daily soil evaporation coe cient (Ke) are considered separately according to the equation: Kcb is a transpiration coe cient and Ke is an evaporation coe cient. The Kcb and Ke are the basal crop and the soil evaporation coe cients. For the dual Kc method, Ks applies only to transpiration (Tr) and provides actual transpiration (Ta). The values of Ks are obtained using its relationship with measured water stress indicators. The FAO procedure for estimating crop consumptive use requirements provides a list of Kc values for each crop and developmental stage. In the Kc approach both crop transpiration and soil evaporation are timely averaged into a single coe cient (Kc) commonly used to obtain the ETc for various crops. The FAO No. 56 Publication offers the option of differentiating E from Tr by using a dual crop coe cient approach.
The estimates of crop water requirements (ETc) are derivable from the product of potential evapotranspiration of a reference crop (ETo) using a crop factor (kc). Based on these relations: The crop coe cient kc is estimated as: The crop coe cient (Kc) 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. (1998) suggested a Kc value of 1.0-1.05 for a cocoa crop with a complete canopy.
Information is inadequate on the actual water use (ETc) of cocoa on the eld (Carr, 2011). Estimated values of ETc ranging from 3 to 6 mm/day during rains and less than 2 mm/day in the dry season have been reported for cocoa (Penman, 1948). Field data (based on the sap ow method) suggest ETc rates of less than 2 mm/day for cocoa crop with a complete canopy, this appear to be low compared with potential ETo estimate of 3-5 mm d −1 using Penman equation (Penman, 1948). . In a simulated El Niˇno drought experiment reported by Moser et al. (2010) in Indonesia, there were no signi cant differences in cocoa leaf, stem and branch wood, or ne root biomass production between the rainfed control treatment and the one in which rain through-fall was reduced by 70-80% (for dry soil pro le 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 ux sensors) was 1. 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 de cit and since irrigation is not part of the farming system, causing seedling mortality (Famuwagun et al., 2017). In bearing plants, the existence of the short dry season during main crop pod lling can affect bean size if it is su ciently severe. In adult plantings, water de cits 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 de cit 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 de cit 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.

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: 7 º 18 I N, Longitude: 5 º 8 1 E and 350 m abs. Five to six years old fruiting cacao trees which had been previously irrigated during dry season from seedling establishment (April, 2013) 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 to over 2000 mm distributed in a bimodal pattern within seven to eight months duration and 3 to 4 months of dry season. The dry season is a terminal drought situation characterized by inadequate rainfall,, soil moisture, high vapour pressure de cit and temperatures stresses and very clear sky (high intensity of solar radiation) (Famuwagun et al., 2017, Charles et al., 2019.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 eld capacity and permanent wilting point are 21 and 10 % respectively. Mean bulk density was 1.25 g cm -3 . The soil at the site of the experiments was Soil samples were taken and subjected to routine Laboratry analysis for physical (textural class, bulk density, water holding capacity) and chemical (organic matter, N, P, K Ca, Mg, CEC, electrical conductivity) properties using standard procedures.
Soil samples were taken using soil Auger for water content measurement within the top soil layer (0 -30 cm) by gravimetric method. Core samples were taken for bulk density and porosity measurement. Soil moisture content would attain eld capacity in two days since the soil is sandy clay to silty clay loam (Agele et al., 2014). 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:(DS) 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 amount was calculated using Pan evaporation and Pan coe cients (Kcp1: 1.0; Kcp2: 0.7, and Kcp3: 0.5) according to Doorenbos and Pruitt (1975) and Allen et al. (1998) as:

Irrigation Strategies
where Ir is the amount of applied irrigation water (mm), A is the plot area, EPan is the cumulative evaporation at irrigation interval (mm) and Kcp is the plantpan coe cient.
Irrigation treatments were coded as EPan *100 Kcp (IrT1), EPan * 70 % Kcp (IrT2) and EPan *50 % Kcp ( IrT3) while irrigation was xed at 5 days-interval for the three irrigation treatments. Irrigation treatment IrT3 had the maximum water de cit 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, 6.8 and 9.6 l/tree/day at each irrigation via point source emitters of 2l/h discharge rate which were installed on laterals per row of cacao tree spaced at 3 x 3 m. One drip lateral served each plant row and an in ow meter was installed at the control unit to measure total ow distributed to all replications in each treatment. Irrigation buckets were suspended on 3. Total amount (volume) of irrigation water applied per treatment was calculated using equation: where, V, is the volume of irrigation water (L); P, wetting percentage (taken as 100 % for row crops); A, is plot area (m 2 ); EPan is the amount of cumulative evaporation for the irrigation interval (5-days) and Kcp Pan coe cients (1.0, 0.7 and 0.5). This corresponded to 7.14 mm (1.93 l/day), 10.7 mm (2.90 l/day), 14.28mm (3.86 l/day) for the respective 0.5, 0.7 and 1.0 Kcp. 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 pro le depth to eld 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 (DI 1 level) to a minimum of 1.93 l/day and 20400 mm (DI 4 level). Irrigations continued until one week before the nal harvest.
where, ET, is actual crop evapotranspiration (mm); I, the amount of irrigation water applied (mm); P the precipitation (mm); ΔSW, changes in the soil water content (mm); Dp, the deep percolation (mm); Rf, 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 coe cient ( Deep percolation was considered as zero because there was no high underground water problem in the area. If available water in the root zone (0-90 cm) and total applied water amount by irrigation were above the eld capacity, it would be assumed that water amount above eld capacity leaked into the deeper soil zones and was called deep percolation (Dp: available total water amount at 0-90 cm soil depth before irrigation + applied irrigation water eld capacity) (Doorenbos and Pruitt, 1977). Total water requirement (WR) was determined using the relation: 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, 6.75 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: Maximum allowable de cit (MAD) for cacao (50% of available water storage capacity of the soil (AWC) Gross irrigation requirement (GIR) of an orchard or vineyard, the computed ETc, which is considered as the net irrigation requirement (NIR), should be divided by the application e ciency (AE).

GIR = NWR/AE …………………………….11
Yield and crop water use were deployed to evaluate appropriate the e ciencies of irrigation management practices among the different irrigation strategies adopted.
Orchard water use e ciencies where IWUE is the irrigation water use e ciency (t.ha 1 mm), EY is the economical yield (t.ha 1 ), Ir is the amount of applied irrigation water (mm).
Cacao water requirement was determined using FAO-56 single and dual crop coe cient models approach. The aim was to analyze the capacity of the FAO-56 single and dual crop coe cient models to assess cacao evapotranspiration and water requirements (estimating adequacy of irrigation amount for cacao) . 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 The ground cover (normally expressed in percentage) was obtained by measuring the shaded area outlined from the horizontal projection of the tree canopy A = d4 2 4 (m 2 )…………………17 d4= diameter of shaded area by cacao canopy ( 2m), A is per cent ground cover by cacao canopy; Tree spacing is 3x3 m (9 m 2 ); d1 (areal canopy area), d2 (height bt d1 and d3); d3 (projection of canopy area on ground, d1> d3). Where G is ground cover fraction of tree canopy, is monthly rainfall amount, wz is fraction of soil surface wetted by drip emitters (ETo = reference ET) Cover crops/weed cover transpiration (Tr cc). where DAF must be > 2); Vu = Vo(dp/10000), Vo = 1/6xD 2 H E (exponent = 2.718), H (height of canopy; m); D is average canopy diameter, m); Vo is canopy volume ; m 3 /tree); Vu is canopy volume as amount on ground cover; m 3 /m 2 ); DAF is leaf area density; dp is tree density; number/ha), Fi = 0.07 for tree density greater than 300 trees/ha), F2 is monthly coe cient of Tr which is about 0.7 to 1.0 from wet to dry seasons Qd =1-e -kextVU Kc, Tr = (QdF 1 ) F 2 ………………………25 ETc = ETo Kc Kr,t………………………26 Kc,t is empirical coe cient relating the ET of an orchard of incomplete cover to a mature orchard of full canopy cover. In addition, Kr,t relates to horizontal projection of tree shade/canopy (ground cover per cent; Orgaz et al., 2006), and Kr,t is about 0 to 70% of G Cacao orchard Transpiration (Tr) was determined as: Tr ETO………………………..27 where Kc, Tr is transpiration coe cient which varies bt 0.75 to 1.0 seasonally until leaf senescence onset for a fully wetted orchards (su cient soil moisture situation) (Kc,Tr 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 (Fig. 1). On the average, the rainy season had higher mean relative humidity averaged (71%) and lower air temperatures (32.8 o 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.
A low pressure gravity-drip system was deployed to deliver water to cacao rootzone which alleviated moisture stress during the dry season.Across sampling dates, different amounts of irrigation water were applied based on cumulative Pan evaporation (EPan)*Pan coe cients for the respective irrigation treatments IrT1 (EPan The irrigation treatments (IrT1, IrT2 and IrT3) affected soil moisture contents within cacao root zone. Soil moisture contents adequately re ected the irrigation water delivered across measurement dates (Fig. 3). Highest soil moisture contents were obtained for well irrigated (IrT1) and lowest for de cit irrigation (IrT3) treatment. For the respective de cit irrigation treatments (IrT2 and IrT3: 0.7 and 0.5 EPan coe cients), average soil moisture contents were 61, 48 and 42% for IrT1, IrT2 and IrT3 irrigation treatments (Fig. 3). Highest soil moisture contents and crop evapotranspiration (ETc) were obtained from well irrigated plots (IrT1: EPan*kcp (1.0 (9.6 l/tree/day) followed by IrT2 (EPan*kcp (0.7) (6.8 l/tree/day) and lowest for IrT3 EPan*kcp (0.5) (4.8 l/tree/day). The de cit irrigation treatments (IrT2 and IrT3) had lower soil moisture contents (14.7 and 11.8% ) which equated to 30 and 50% water savings.
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 de cits). Unfavourable weather of high temperatures and soil evaporation and low atmospheric humidity would enhance soil moisture depletion thus the low soil moisture status (Agele et al., 2011, Agele 2021. 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 rootzone 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 -20 cm soil pro le depth before and one day after each irrigation. Soil moisture contents across measurement days ranged between wilting point (140 mm) before irrigation and eld 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. For the de cit irrigation treatments (IrT2 and IrT3: 0.7 and 0.5 Pan coe cients), available water fell below 50% more often than not during the period of study. Because much more water was applied under high Pan coe cients (Kcp 1.0), soil moisture contents of well watered treatments (IrT1) was higher compared with de cit irrigation (IrT2 (0.7 Kcp: and IrT3 0.5 Kcp) treatments. The well watered treatment (IrT1), most times, maintained soil moisture within eld capacity range.
In general, based on the values of soil moisture, the stored water within crop rootzone pro le was used up between irrigation cycles. This is attributable to the intensities of climatic stress (high temperatures and vapour pressure de cits) which presumably enhanced soil evaporation and the rapid depletion of water stored in the soil pro le. Soil moisture content immediately following irrigation gradually decreased towards next irrigation event, this situation con rms the inability of soil moisture reserve to satisfy cacao water demand during the dry season which was consistent with earlier reports of Soil moisture depletions over two measurement days were deployed to determine cacao water use (ETc). Cacao water use (ETc) differed across measurement dates and irrigation treatments (Fig. 4a). Average cacao evapotranspiration (ETc) were 139, 97and 63 mm/day for the respective IrT1 (IrT1 (Kc:1.0), IrT2 (Kc 0.7) and IrT3 (Kc 0.5). Cacao evapotranspiration (ETc) for the de cit irrigation treatments (IrT2: 0.7 and IrT3:0.5 EPan coe cients) were averagely 45 and 70% less compared to values to adequate irrigation (IrT1) treatment which signi ed soil moisture de cit stress. Peak ETc values were obtained at DOY 45, 60 and 75 possibly due to high EPan (> 5 mm/day), lowest for DOY 120 and 135 with increases afterwards. The increases in cacao water use (ETc) 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 ETc and the more de cit irrigation (IrT3) had least cacao water use (Fig. 4a). The mean ETc across measurement dates were 5.07, 3.55 and 2.63 mm/day for IrT1, IrT2 and IrT3 irrigation treatments for the period of experiment (December to May).
In addition to single crop coe cient (kc = 1.31), cacao water requirement (ETc) was also computed using the dual (kr t: 1.04) crop coe cient (Fig. 4b). Means of cacao water use for dual crop coe cient across measurement dates were 5.2, 3.7 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 coe cients are presented in Fig. 5a, b and c. Results showed similar trends in cacao ETc for both methods and irrigation treatments while values were higher for the dual coe cient compared with the single kc (Fig. 5a, b and c). The decreasing order of ETc for single kc and dual kc were IrT1 > IrT2 > IrT3. Crop evapotraspiration (ETc) increased with increases in the volume of irrigation water applied, this modi ed values of ETc obtained for both the single and dual kc approaches.
The FAO-56 dual crop coe cient approach (Allen et al., 1998) which describes the relationship between crop evapotranspiration (ETc) and reference evapotranspiration (ETo), separates the crop coe cient (Kc) into the basal crop (Kcb) and soil water evaporation (Ke) coe cients. In the single crop coe cient approach, the effect of both crop transpiration and soil evaporation are integrated into a single crop coe cient (Kc) while in the dual coe cient approach, a daily basal crop coe cient (representing plant transpiration: Kcb) and daily soil evaporation coe cient (Ke) in the form of Kc = Kcb + Ke). However, crop evapotranspiration (ETc) estimation is more accurate by dual crop coe cient approach than the single crop coe cient approach, the dual crop coe cient approach uses more parameters and take soil management practices and crop characteristics into consideration (Allen et al., 1998).
The magnitudes of cacao ETc (single and dual crop coe cients obtained from the respective irrigation treatments followed from the irrigation water delivered (Fig. 5a, b and c). 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 de cit irrigation treatments (IrT2 and IRT3) which is consistent with reports on citrus by Yang et al.  (Penman, 1948, Moser et al., 2010. Field data (based on the sap ow method) suggest ETc rates of less than 2 mm/day for cocoa crop with a complete canopy, this appear to be low compared with potential ETo estimate of 3-5 mm d −1 using Penman equation (Penman, 1948).
The ETc/EPan ratio denotes the proportion of climatic water demand satis able by crop water use (ETc). Among the irrigation treatments. The proportions of Pan evaporation (EPan) to cacao water use (ETc) denoted as ETc/EPan ratio, differed across measurement dates and irrigation treatments. The means of ETc/EPan ratios across measurement dates were 1.016, 0.714 and 0.492 for the respective IrT1, IrT2 and IrT3 treatments (Fig. 6a). ETc/EPan curves were similar but the ratios ranged from 1.16 to 0.50 which indicated that both climatic demand (EPan) and cacao water consumption (ETc) were high during the dry season at the site of study. Although a weak relationship, linear regression equation was tted to the ETc/EPan trends as: Y = 0.011x+0.94, R 2 = 0.32). The ratio of water use (ETc) to irrigation water applied denotes the proportion of irrigation water applied used for crop evapotranspiration. Trends of ETc to irrigation were similar among irrigation treatments and measurement dates but values differed among irrigation treatments (Fig. 6b). The mean values were 1.016, 0.714 and 0.492 for the respective IrT1, IrT2 and IrT3 treatments (Fig. 6b). 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 (EPan) driven trends of cacao water consumption (ETc). 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 andKassam, 1979, Allen et al., 1998). Doorenbos and Kassam (1979) found positive linear and signi cant logarithmic correlation (P < 0.01) between ETc and EPan while Smajstrla et al. (2000) obtained signi cant logarithmic correlation (P < 0.01) between ETc and EPan. These reports con rmed the established close relationship between plant water consumption and Pan evaporation.

Soil evaporation from cacao orchard
The ETc from an orchard is more complex. In addition to tree Tr, there could be Tr 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 (Edz) and the non-wetted zone (Ewz) during the period of experiment. The mean values for soil evaporation for the wetted (Edz) were 5.65, 2.82 and 0.19 mm/month for the respective IrT1, IrT2 and IrT3 treatments. The seasonal totals soil evaporation for the wetted (Edz) and the non-wetted ( Ewz) zones were 234.29 and 33.94 mm respectively. Based on the cumulative seasonal totals, soil evaporation for the wetted zone (Edz) was averagely 7 times compared with the non-wetted zone( Ewz) (Fig. 7).
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 (Edz) and the non-wetted (Ewz) zones within cacao eld (Tombesia et al., 2018). 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 Tr and Es in orchards (Bonachela, 2001, Tombesia et al., 2018). 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). 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 ETo from the wet areas (Bonachela, 2001, Garcia-Tegera et al., 2017). As a rst approximation, the quanti cation of E from the wetted spots in a drip-irrigated orchard can be made using a semi-empirical model of Bonachela (2001)    Cocoa is cultivated as a rainfed crop but sensitive to weather extremes of low rainfall, soil moisture de cit and high temperature stresses had been variously reported in the literature ( 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 de cits) 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.       Please See image above for gure legend.