Smallholder Irrigation for Climate Mitigation and Cacao (Theobroma cacao L.) Performance Improvement in the Rainforest Tropics

Abstract

Climate change poses significant threats to agriculture, including food security, livelihoods and economic growth. Based on the importance of cocoa, there is a need for sustainable crop production and resilience to anticipated changes in rainfall and temperature in the future. Irrigation is an important climate-smart practice for alleviating abiotic stress and enhancing crop productivity, and irrigation is seldom practiced in the cacao orchards of West Africa. Studies were conducted to examine the effects of dry season gravity drip irrigation on the rootzone moisture, tree water use (evapotranspiration), leaf area index and yield of cacao in a rainforest zone of Nigeria. Irrigation treatments were based on water application at 5- and 10-day intervals and 50, 70 and 100% Pan evaporation, which was applied using point source emitters on drip lines. The soil moisture content, photosynthetic active radiation, leaf area index and extinction coefficient differed among the irrigation treatments. Deficit irrigation (10-day and 50% EPan) enhanced water use efficiency by 25–44% (30 and 50% water savings), while full irrigation enhanced soil moisture, cacao ET, and pod and bean yields. This study established irrigation and water requirements for cacao in the dry season and confirmed the relevance of irrigation for enhanced cacao performance and climate mitigation.

1. Introduction

Cocoa (Theobroma cacao L.) is an important perennial fruit tree with an estimated annual global production of 3.2 million tonnes [1]. In West African cocoa-producing countries, cocoa is a major foreign exchange earner and provides employment for millions of smallholder farmers whose small farm sizes range from 0.5 to 5.0 ha. In Nigeria, the main cocoa-producing areas are in the rainforest south where an estimated.

A total of 1.45 million hectares are cultivated, and cocoa productivity is low (approximately 250 kg dry beans/ha) compared with 600–1000 kg/ha for Cote d’Ivoire and Indonesia.

Cocoa is the most prominent perennial fruit tree species in the rainforest of Nigeria. Fruit trees are characterized by deciduous growth habits but are cultivated under rainfed conditions [2, 3]. The annual rainfall in the cocoa growing region of West Africa has a bimodal distribution of less than 2000 mm, resulting in wet–dry seasonal transitions [2, 4]. The dry season lasts approximately 3–4 months and is characterized by dry and hot weather. Smallholder farmers rarely practice irrigation; nevertheless, irrigation is effective for meeting crop water demand and ameliorating climate-related stresses [5, 6]. In West Africa, cocoa is cultivated as a rainfed crop and is thus subjected to poor soil and weather conditions [3, 5, 7]. Cocoa is cultivated as a rainfed crop, and it is highly sensitive to soil and weather conditions such as low rainfall, soil and air moisture deficits and temperature stresses [3, 5, 7]. The changing environmental conditions (marginal soils and extreme weather events) impose constraints on cacao growth and productivity. The annual rainfall in the cocoa growing region of West Africa follows a bimodal rainfall distribution pattern of less than 2000 mm (wet–dry seasonal transitions: [2, 4]). During the 3 months of dry and hot weather, smallholder farmers rarely practice irrigation. However, irrigation is effective for meeting crop water demands and ameliorating climate-related stresses [6].

Climate change, extreme and variable weather conditions and other climate-related disasters, such as high frequency and severe drought events and warming temperatures (1.5–2°C), occur worldwide [8]. These changes may result in changes in regional precipitation and evapotranspiration [9, 10]. Among natural hazards, drought ranks first in terms of the number of people directly affected [11].

Climate mitigation may be built on adaptation practices to strengthen the resilience of farming systems to anticipated changes in rainfall patterns and temperature in the future. Climate-smart cocoa production landscapes may be built on agricultural innovations such as high-yielding drought-tolerant crop varieties, climate information services, agricultural insurance, nutrient and water management via irrigation, mulching, etc. Sustainable management practices for cacao (Theobroma cacao L.) in a changing climate may include irrigation schemes for climate stress alleviation and enhanced productivity.

The available information on cocoa water use in the field showed estimated values ranging from 3 to 6 mm/day during rains and less than 2 mm/day in the dry season [12, 13]. The FAO Penman–Monteith equation is accepted worldwide as the standard method for estimating reference evapotranspiration (ETo). The ETo indicates crop consumptive water use as the sum of evaporation from soil and plant transpiration f [13, 14, 15, 16]. FAO Paper No. 56 highlights procedures to calculate ETo from radiation, wind, humidity and temperature data in addition to the crop coefficient K_c for crops [17]. Allen et al. [17] suggested a K_c value of 1.0–1.05 for a cocoa crop with a complete canopy. Field reports of cocoa water use (ETc) have shown values ranging from 3 to 6 mm/day during rains and less than 2 mm/day in the dry season [12, 13], while data based on the sap flow method suggest values less than 2 mm/day. Moser et al. [18] conducted a simulated El Niňo drought experiment and reported no significant differences between a rainfed control treatment and rain through-fall reduction (70–80% under a dry soil profile near the permanent wilting point). The reports showed that the maximum cacao bean yields were obtained for cocoa drip irrigated with 20 l/tree^/day (175 l/tree as total irrigation). Based on field trials, Diczbalis et al. [19] reported an annual irrigation requirement of 470 mm for cacao (a maximum weekly requirement of approximately 200 l/tree (1250 trees/ha)) and dry bean yields between 1.5 and 2.7 t/ha for young fruiting trees.

Information on soil moisture extraction, water use and cocoa yields under dry irrigation from the rainforest belt of West Africa is inadequate. Thus, experiments were designed to examine the effects of regulated dry season irrigation on the root zone moisture, tree water use and bean yield of cacao in a rainforest zone in Nigeria.

2. Materials and methods

2.1 Experimental site and conditions

Experiments were conducted in the field using 5-year-old cacao trees that had been previously irrigated since the first year of field establishment (2012). The study was carried out at the Research Station of the Federal University of Technology, Akure, in the rainforest zone of Nigeria. In agroecology, rainfall patterns are characterized by bimodal and seasonal wet–dry transitions. The dry season is characterized by terminal drought caused by inadequate rainfall, soil moisture deficits, high vapor pressure deficits and temperatures and very clear skies [20].

2.1.1 Irrigation strategies

The drip irrigation system (drip irrigation) was laid out in the field. This included a pumping machine, good water source, pipes, drip lines, overhead tank (with stand), and pressure control valves. Water was applied via a gravity-drip irrigation system via point source emitters, which were installed on the laterals of each row of crops. The emitters were installed on the laterals of each row of crops and were spaced 3 m apart. The irrigation buckets were suspended on 5 m high tank stands to provide the required hydraulic heads [6]. The low-head (gravity) drip system supplied water to plant roots via drippers using inline emitters with a discharge rate of 2 l/h, which were spaced at 3 m intervals laterally. One drip lateral served each plant row. An inflow meter was installed at the control unit to measure the total flow distributed to all replications in each treatment.

2.2 Experiment 1: irrigation treatments at 5- and 10-day intervals

Preliminary studies based on variable irrigation amounts and frequencies for cacao in the study area have shown promising results (Agele, personal communication) [2]. The present study is a follow-up study aimed at validating the split-application of 14.28 mm (3.86 l/day) at 5- and 10-day irrigation intervals in the field. A pretreatment of 135 mm of irrigation water was applied to replenish the soil water within a 0.60 m profile depth (to field capacity). Irrigation was applied at 5- and 10-day intervals and was arranged in a split-plot design.

Irrigation was imposed based on the restoration of cumulative potential evapotranspiration (ETo) via the FAO method [17, 21] in the following form:

$$\mathrm{ETa}=K_{\mathrm{c}}\mathrm{ETo}$$E1

where ETo is the potential evapotranspiration and K_c is the crop coefficient [17, 21].

A crop coefficient (K_c) of 0.83 was adopted for cacao (which was in the early fruiting stage) [17].

The potential evapotranspiration (ETo) from December to May was derived from the Penman–Monteith combination equation [17, 21] using data obtained from the Meteorological Observatory of the Experimental Station.

The water requirement (WR) was determined using the following relation:

$$\mathrm{WR}=A\ast B\ast C\ast D\ast E$$E2

where WR = water requirement (l day/plant), A = open Pan evaporation (mm/day), B = Pan factor (1.0), C = spacing of plant (m²), D = crop factor. The crop factor depends on plant growth; the value for fully grown cacao was 1.13, but for cacao in the early fruiting stage, 0.83 was adopted.

The irrigation amount (volume per application) was calculated as follows:

$$V=P\ast A\ast E_{\mathrm{Pan}}\ast K_{\mathrm{cp}}$$E3

where V is the volume of irrigation water (l); P is the wetting percentage (taken as 100% for row crops); A is the plot area (m²); and E_Pan is the pan evaporation and K_cp pan coefficient (1.0). This corresponded to 14.28 mm (3.86 l/day), an amount that was applied at 5- and 10-day intervals.

The irrigation WR was determined using seasonal Pan evaporation data for the area. The total water requirement (TWR) of the farm plot was obtained as follows:

$$\mathit{TWR}=\mathit{WR}\ast \mathit{\text{Number of plants}}$$E4

where TWR is the total water requirement and WR is the water requirement (l day/plant).

The maximum allowable deficit (MAD) for cacao was assumed to be 50% of the available water storage capacity of the soil (AWC).

The actual evapotranspiration (consumptive water use: ETc) of cacao trees was derived from the water balance equation (Eq. (1)) [6].

$$\mathrm{ET}=I+P+\mathrm{dS}-\mathrm{Dp}-\mathrm{Rf}$$E5

where ET is the actual crop evapotranspiration (mm) and I is the amount of irrigation.

Water applied (mm); P, precipitation (mm); dS, change in the soil water content (mm); Dp, deep percolation (mm); Rf, amount of runoff (mm). Since the amount of irrigation water was controlled, deep percolation and runoff were assumed to be negligible.

Soil water measurements were taken throughout the growing season using the gravimetric method.

The maximum (management) allowable deficit (MAD) for cacao was set at 50%.

Cacao LAI and solar radiation integrals (incident, transmitted and absorbed radiation, the ratio of radiation measurements below and above the canopy and PAR) were measured using a canopy analyzer (Delta T, UK). The incident solar radiation (R_I) above the canopy was measured using a pyranometer connected to the canopy analyzer system.

The line sensor was attached to a metal frame and lifted above the cacao canopy.

Photosynthetic active radiation (PAR) was measured in addition to solar radiation. The analyzer measures light transmitted by the ratio of radiative measurements below and above the canopy [20]. The fraction of intercepted radiation (R_fract, %) was calculated as follows:

$$R_{\text{frcat}}=(R1-R_{\mathrm{T}}/R_{\mathrm{I}}$$E6

where I is the incident radiation above the canopy and T is the transmitted radiation.

Canopy extinction coefficient.

The Beer–Lambert law describes the absorption of light by plant pigments in solution. This function demonstrates that the absorption of light will be more or less exponential with increasing intercepting area down through the canopy. The light extinction coefficient (k), according to the Beer–Lambert law, is:

$$k=\left[\text{loge}\left(\frac{I}{\mathit{Io}}\right)\right]/\mathrm{LAI}$$E7

$$k=-ln\left(I-I_{\mathrm{o}}\right)/\mathrm{LAI}$$E8

where I and I_o are the irradiance values upon and under the canopy, respectively.

LAI is the LAI of leaves causing light attenuation, and k is the extinction coefficient or slope of the curve when the natural log (In) I/I_o is plotted against the LAI. The light extinction coefficient (k) was calculated by inverting Lambert–Beer’s law Taku et al. [22]:

$$K\mathrm{df}=-ln(0.94\mathrm{PAR}\text{transmitted}/\mathrm{LAI}$$E9

Fractional radiation (I) interception was calculated according to the following equation:

$$I=\left(R_{\mathrm{i}}-R_{\mathrm{t}}\right)/R_i$$E10

where R_i is the incident radiation and R_t is the transmitted radiation.

The proportion of transmission (TR) from the incident radiation (Ra) was obtained by the following formula:

$$\mathrm{TR}\left(\%\right)=\left(\frac{\mathrm{Rb}}{\mathrm{Ra}}\right)\ast 100$$E11

2.3 Experiment 2: irrigation treatments with 50, 70 and 100% pan coefficients (K_cp)

The irrigation treatments were based on variable Pan coefficients and were delivered using point source emitters on gravity-powered drip lines installed on rows of trees [6, 17, 23]. Pan coefficients (100, 70 and 50% K_cp amount to 0, 0.3 and 0.5 relative water deficits, respectively). Thus, the amount of irrigation water to be delivered was derived from the product of Pan evaporation and Pan coefficients (K_cp) [17, 21]. The adopted coefficients were 1, 0.7 and 0.5, respectively:

$$\mathrm{Ir}=A^{\ast }{E_{\mathrm{Pan}}}^{\ast }{K}_{\mathrm{cp}}$$E12

where Ir is the amount of applied irrigation water (mm), A is the plot area, and E_Pan is the cumulative evaporation at the irrigation interval (mm) and K_cp are the plant-pan coefficients. The irrigation treatments were IrT1 (E_Pan * 100 K_cp) and IrT2 (E_Pan * 70% K_cp) and IrT3 (E_Pan * 50% K_cp).

These treatments denote the adequacy of water delivery (IrT1: the noncrop water stress baseline), and IrT3 denotes the maximum water deficit (the stressed baseline).

The actual crop evapotranspiration (ETc) of the cacao trees was calculated according to Eq. (5), and the TWR was determined according to Eq. (2) (Experiment 1). Therefore, the irrigation requirements (WRs) were 9.63, 6.75 and 4.82 l/plant/day for the IrT1, IrT2 and IrT3 irrigation treatments, respectively. The seasonal irrigation amount ranged from 4.82 l/day and 127,500 mm (at the DI₁ level) to a minimum of 1.93 l/day and 20,400 mm (at the DI₄ level). The irrigation WR and TWR of the farm plot were obtained according to Eq. (4) (Experiment 1).

The gross irrigation requirement (GIR) of the coca field (computed as ETc) is considered the net irrigation requirement (NIR), which is obtained following its division by the application efficiency (AE).

$$\mathrm{GIR}=\mathrm{NWR}=\mathrm{AE}:$$E13

2.4 Orchard water use efficiencies

Crop water productivity (efficiencies of crop water use WUE) and irrigation treatments (irrigation use efficiency: IWUE) were determined using the methods of Sezen et al. [23] and Agele et al. [6]:

$$\text{IWUE}=\text{biomass weight}\left(Y\right)/\text{total irrigation water applied}\left(\mathrm{Ir}\right)$$E14

$$\mathrm{WUE}=\text{biomass weight}\left(Y\right)/\text{cumulative seasonal}\mathrm{ETc}$$E15

where IWUE is the irrigation water use efficiency (t/ha/mm) and EY is the economic yield (t/ha), and Ir is the amount of applied irrigation water (mm).

3. Results and discussion

3.1 Effects of 5- and 10-day irrigation intervals on cocoa production

Compared with the dry season (December to March), the rainy season (March to early December) had a higher mean relative humidity average (71%), high cloud overcast (overcast sky) and lower air temperatures (32.8°C). The cumulative amounts of seasonal irrigation water delivered were 12,119, 8483 and 6059.3 mm (Figure 1). Compared with the 10-day interval, the 5-day irrigation treatment resulted in greater water delivery to the cocoa root zone across the sampling dates. The soil moisture contents differed among the irrigation water deliveries and measurement dates, and higher soil moisture contents were obtained for the 5-day irrigation interval than for the 10-day irrigation interval (Figure 2). On average, the soil moisture content was 48% greater for the 10-day irrigation treatments (Figure 3) than for the 5-day irrigation treatments. The lowest soil moisture contents were obtained for DOY 45 and 120, and the highest soil moisture contents were found between DOY 345 and 75 and between DOY 120 and 150 (Figure 2). There were significant differences (P < 0.05) in soil moisture between the 5- and 10-day irrigation intervals, while the 5-day intervals of irrigation delivered more irrigation and enhanced the soil moisture status compared with the 10-day intervals. The time course of the status of soil water before and after irrigation using moisture content measurements from soil samples within the 0–20 cm soil profile depth before and 1 day after each irrigation. At times, both irrigation intervals had values close to the wilting point between irrigation events, under which available water fell below 50% more often than not during the period of study. With more frequent irrigation (i.e., 5 days of irrigation), the soil moisture was mostly within the field capacity range. In general, based on the values of soil moisture, the stored water within the crop rootzone profile was used between irrigation cycles.

Figure 1.

Seasonal irrigation water was applied (at 5- and 10-day irrigation intervals).

Figure 2.

Effect of irrigation on the soil moisture status.

Figure 3.

Cacao evapotranspiration (ETc) as affected by irrigation intervals.

Soil moisture depletion over two measurement days was analyzed to determine the soil water use (ETc). Cacao water use (ETc) differed across measurement dates and irrigation treatments (Figure 3). The mean calculated evapotranspiration (ETc) values were 3.72 and 3.54 mm/day, 3.44 and 3.15 mm/day, and the seasonal totals were 48.2 and 38.3 for the 5- and 10-day irrigation intervals, respectively. The average Cacao evapotranspiration (ETc) for the 10-day irrigation treatment was 45% less than that for the 5-day irrigation treatment. The time course of cacao evapotranspiration (ETc) showed that cacao water use declined between DOY 345 and 45, while the lowest values were found for DOY 45 to 105.

for both 5- and 10-day irrigation intervals (Figure 3). Pod and bean yields were significantly different under the 5-day irrigation treatment, which produced more and heavier pods and beans than under the 10-day irrigation treatment. The pod and bean weights for the 5- and 10-day irrigation treatments were 78,000–6000 kg/plant and 4.8–3.2 t/ha, respectively, and the water productivities were 0.45–0.33 mm/kg/ha (irrigation efficiencies) and 0.11–0.09 mm/kg/ha (crop water use efficiencies), respectively (Table 1).

Irrigation	Seasonal irrigation	Soil moisture content (%)	ETc	Seasonal ETc	Pod weight/plant (kg)	Bean weight/Plant (g)	Bean yield (ha)	WUE (bean) (Irr)	WUE (bean) (ETc)	PAR	LAI	K
5-Days	12,452	19.2	3.71	43.85	7478	816.6	4.59	0.043	0.101	628.1	3.51	0.61
10-Days	6226.2	16.4	3.23	41.44	6352	708.3	3.53	0.035	0.083	585.7	3.14	0.57
LSD (0.05)		1.9	0.81	1.14	0.94	115.4	1.06	0.007	0.003	61.3	0.31	0.03

Table 1.

Summary of measured soil and cacao parameters as affected by 5- and 10-day irrigation intervals.

Solar radiation integrals (the ratio of transmitted to incident radiation, PAR and LAI), the cacao canopy cover (LAI) and PAR intensities and the canopy extinction coefficient (k)) were greater under the 5-day irrigation interval than under the 10-day irrigation interval. AnimKwampong and Frimpong [4] and Agele et al. [20] reported that canopy size and resultant shade intensities affect light characteristics within cacao fields, particularly the ratio of transmitted to incident radiation, PAR and LAI [2, 24]. Anim-Kwampong and Frimpong [4] suggested LA1 > 1 as the threshold canopy cover of the land surface for optimum light interception and transmission. A relatively high LA1 will result in mutual shading, limiting light transmission and photosynthetic activity. The Cacao LAI affects light attenuation and other radiation characteristics within the canopy, such as an increase in diffuse light [25]. Diffuse radiation is associated with increases in canopy light attenuation (canopy extinction coefficient). The canopy extinction coefficient (k) was greater for the 5 days of irrigation, which also had a greater LAI. The higher extinction coefficient for the 5-day irrigated cacao may be due to the lower amount of transmitted light (diffuse radiation). Goudriaan and Monteith [26] affirmed that a low extinction coefficient enhanced growth when the plant canopy was fully developed and uniformly distributed. Acheampong et al. [27] reported that biomass accumulation and overall development of cacao depend on the intensity of the PAR received. Irrigation ameliorated the microclimate via a reduction in thermal load and improved the soil moisture status and crop evapotranspiration [2, 18]. Such modification of the microclimate would enhance CO₂ assimilation by leaves and Charles et al. [2] reported that a large extent of tree canopy will increase vegetative growth and photosynthetic activity of leaves. Reduced hydrothermal stress has implications for cacao survival and productivity during the terminal drought situation of the dry season in the study area.

Cacao irrigation and WRs have been variously studied [12, 13, 18, 19, 28]. Cacao water use values were reported to range from 1.3, 1.15, 3 and 5 mm/day using E_Pan coefficients between 1.0 and 0.6 and 224 mm cumulative seasonal water use during the dry season and weekly irrigation requirements of 470 mm and 200 l/tree. Penman [13] computed 3–5 mm/day as an ETc and an ETc of approximately 1.3 mm (10 l/tree/day), Kohlerlscher et al. [28] reported 2 mm/day, and Moser et al. [18] reported 1.3–1.5 mm/day. In Cote d’Ivoire, the depth of irrigation of 920–1650 mm and E_Pan coefficients between 1.0 and 0.6 and 224 mm cumulative cacao water use during the dry season produced between 30 and 60% of the cacao bean yield increase [19]. The authors applied 470 mm of seasonal irrigation and 200 l/tree for weekly irrigation, resulting in bean yields of 1.5–2.7 t/ha. The present obtained cacao yields ranged between 7800 and 6000 kg pods/plant and between 430 and 280 g beans/plant (4.8 to 3.2 t/ha). The more frequent irrigation out-yielded deficits of 20 and 24% for pods and beans, respectively. More frequent irrigation (at 5-day intervals) than at 10-day intervals enhanced the soil moisture content and water use (ETc).

3.2 Effects of irrigation using pan coefficients (Kcp) of 1.0, 0.7 and 0.5 on cocoa production

The irrigation regimes computed using 100, 70 and 50% Pan coefficients affected the soil moisture content, water use, pod and bean yields and water productivity (irrigation and crop evapotranspiration) of cocoa. Irrigation with Pan coefficients (Kcp) of 1.0, 0.7 and 0.5 delivered different amounts of water to the cacao rootzone. The irrigation amounts (monthly averages) were 1009.88, 706.91 and 504.94 mm, while the seasonal totals were 2116.5, 8482.95 and 6059.25 mm for the IrT1, IrT2 and IrT3 treatments, respectively (Figure 4). Irrigation at IrT1 (E_Pan * 100 K_cp) had the greatest effect, and irrigation at IrT3 (E_Pan * 50% K_cp:0.5) had the least effect. Compared with IrT1, IrT2 and IrT3 delivered 79 and 68% more water to the cacao rootzone, respectively. The maximum irrigation amount occurred on DOY 45, 60, 75 and 90, which coincided with the highest E_Pan values (>5 mm/day).

Figure 4.

Seasonal irrigation delivery (IrT1, IrT2 and IrT3).

Irrigation treatments (IrT1, IrT2 and IrT3) affected the soil moisture content within the cacao root zone. (Figure 5). The highest soil moisture content was obtained for the well-irrigated treatment (IrT1), and the lowest was obtained for the deficit irrigation treatment (IrT3). For the respective deficit irrigation treatments (IrT2 and IrT3: 0.7 and 0.5 Pan coefficients), the average soil moisture contents were 61, 48 and 42% lower than those in IrT1, respectively, and irrigation equated to 30 and 50% water savings (Figure 5). The decreasing trends of the calculated evapotranspiration (ETc) were IrT1 (9.6 l/tree/day) > IrT2 (6.8 l/tree/day) > IrT3 (4.8 l/tree/day).

Figure 5.

Soil moisture contents as affected by irrigation treatments.

Decreases in soil moisture contents were obtained from DOY 345 to 60, followed by increasing trends in soil moisture from DOY 75 until the end of the measurement (DOY 150). The decreasing trends in the values of soil moisture content may be attributed to the increasing intensities of climatic demand (high vapor pressure deficits).

Unfavorable weather, high temperatures, soil evaporation and low atmospheric humidity enhance soil moisture depletion and thus decrease the soil moisture status [5, 29]. In general, the observed trends in the status of rootzone moisture are attributable to the prevailing weather conditions, which are denoted by increasing intensities of climatic demand (vpd) and temperatures during periods (DOY 345 to 60) of the experiment. An increase in moisture was observed from DOY 75 until the end of the experiment (DOY 150) can be attributed to rainfall received following its commencement (mid-March). In general, the soil moisture within the crop rootzone profile decreased between irrigation cycles. This may be attributed to the intensities of climatic stress (high temperatures and vapor pressure deficits), which presumably enhanced soil evaporation. The soil moisture reserve was unable to meet the cacao water demand during the dry season [2, 30].

Cacao water use (ETc) was determined from two soil moisture measurement cycles. Cacao water use differed across measurement dates and irrigation levels (Figure 6). The average calculated evapotranspiration (ETc) values were 139, 97 and 63 mm/day for the IrT1 (K_c:1.0), IrT2 (K_c 0.7) and IrT3 (K_c 0.5). Cacao evapotranspiration (ETc) under deficit irrigation (IrT2 and IrT3) was 45 and 70% lower than that under IrT1 (100 K_cp). The observed differences in ETc values during the experimental periods are attributable to changes in weather conditions. Following the commencement of rainfall and associated replenishment of soil moisture, lowering of temperatures (air and soil) and high atmospheric humidity (declining atmospheric demand), high amounts of cacao water were obtained.

Figure 6.

Cacao water use (evapotranspiration; ETc).

Yang et al. [31] recorded higher soil moisture status and cacao ETc for irrigated citrus plants using full-pan evaporation. The magnitude of the calculated ETc was also within the range reported by Carr [12]. Penman [13] obtained 3–5 mm/day during rains and less than 2 mm/day in the dry season for cacao under an irrigation regime of 10 l/tree/day. Similarly, Kohlerlscher et al. [28] reported an ETc of 2 mm/day, while Moser et al. [18] reported an ETc of 1.3–1.5 mm/day. A field study using sap flow sensors reported 2 mm/day for cacao water use, which is lower than that in earlier reports. For example, Penman [13] reported a potential ETo estimate of 3–5 mm/day using the Penman equation. Irrigation replenished soil moisture depletion, while cacao shade offered soil surface cover, creating a favorable microclimate for the trees and consequently reducing soil evaporation [9]. Studies have reported that the conditions at the soil surface are affected by wetting via irrigation (amount and irrigation intervals) in addition to soil exposure to light [31, 32, 33, 34]. Soil surface conditions are known to determine the magnitude of crop water use (evapotranspiration: ETc) in orchards [9, 29].

The irrigation regime affected the pod and bean yields of the cacao plants. The values were significantly greater for IrT1 (35.4 and 2.29 t/ha) than for IrT2 (22.1 and 1.37 t/ha) and IrT3 (10.3 and 1.03 t/ha); thus, bean yields decreased by 60 and 40%, respectively, under IrT3 and IrT2 (Table 2). Carr [12] and Charles et al. [2] reported that in addition to irrigation, the yield of cacao also depends on soil properties such as infiltration rate and water holding capacity. Other studies have reported the effects of irrigation on the biomass, pod and bean yields of cacao. Diczbalis et al. [19] applied seasonal irrigation of 470 mm weekly at 200 l/tree and obtained bean yields of 1.5–2.7 t/ha.

Irrigation	Seasonal irrigation	Scm (%)	ETc single (kc:1.13)	ETc Dual (kr t: 1.04)	ETc/EPan ratio	Seasonal ETc	Pod weight/plant (kg)	Bean weight/plant (g)	Bean yield (kg/ha)	WUE (Irr)	WUE (ETc)
IrT1	33858.2	21.4	5.07	5.2	0.92	139.1	4429	396.5	4.41	0.0117	0.032
IrT2	32705.3	17.3	3.55	3.7	0.73	97.3	3125	334.3	3.72	0.0142	0.043
IrT3	16929.4	14.4	2.63	2.8	0.56	62.7	2673	308.1	3.42	0.0182	0.055
LSD (0.05)		4.1	1.8	1.6	0.21	17.3	112.3	23.8	0.25	0.003	0.005

Table 2.

Summary of measured soil and cacao parameters as affected by irrigation at 100, 70 and 50% pan coefficient (K_cp).

IrT1 (irrigation @ 100% K_cp), IrT2 (irrigation @ 70% K_cp) and IrT3 (irrigation @ 50% K_cp).

The effects of the irrigation regime on the water productivity of cacao were evaluated. The ratio of yield to evapotranspiration (Y/ETc) ranged between 0.3 and 0.04 t/mm, while the yield to irrigation amount (Y/Irrig) ranged between 0.16 and 0.19 kg/mm (Table 2).

This confirmed the superiority of deficit irrigation in terms of water savings (30 and 50%) over the well-watered treatment.

4. Conclusions

Dry season irrigation enhanced soil moisture status, tree water use, canopy characteristics, pod and bean yield of cacao in a rainforest zone of Nigeria. Solar radiation properties (transmission through the cacao canopy, photosynthetic active radiation (PAR)) and canopy area and light attenuation (extinction coefficient, k) differed among the irrigation treatments. Compared with the 10-day irrigation interval, the 5-day irrigation interval enhanced the soil moisture status, cacao water use (ETc), and pod and bean yields of cacao. The deficit irrigation treatments (10-day intervals, especially irrigation at 50% Pan coefficient) increased the water use efficiency (25–44%), which translated to 30 and 50% water savings, respectively.

Smallholder gravity drip irrigation can ameliorate climate stress and enhance cocoa performance and is recommended for scaling-up. This study established irrigation and WRs using variable irrigation intervals and Pan coefficients for cacao during the dry season in the rainforest zone of Nigeria. These findings will inform water management decisions for optimizing the yield and water productivity of cacao, especially in the era of warming and drought. The water-saving advantage of deficit irrigation strategies can be scaled up for adoption.