The field experiment was conducted during the 2011 dry season at the Irrigation Research Fields of the Institute for Agricultural Research (IAR), Samaru, Nigeria, to evaluate yield and soil water balance of two cowpea varieties under deficit irrigation and mulch using locally assembled weighing- type micro lysimeter. The treatments comprised of three levels of irrigation water application depths (50%, 75% and 100% of weekly reference evapotranspiration (WRETo)) and two levels of mulch (No mulch-NM, and black polyethylene mulch -BPM) and two varieties of cowpea (SAMPEA7 and 9) laid in a group balanced block on split-plot design. The result showed that the yield of the cowpea pods ranged from 0.29 t/ha to 1.29 t/ha. The highest yield was obtained from SAMPEA7 treatment irrigated at 100% WRETo with BPM. The drainage depth ranged from 3.9 mm to 89.5 mm with the least value obtained by SAMPEA9 at 50% WRETo and NM. The highest depth was by SAMPEA9 at 100% WRETo and BPM. In comparison, SAMPEA7 gave less drainage water compared to SAMPEA9. The study showed that crop water use of cowpea ranged from 187.6 mm to 335.6 mm for SAMPEA9 and 191.3 mm to 315.8 mm for SAMPEA7 with the least values occurring at 50% WRETo and NM. The estimated values of Kc for the two cowpea varieties at initial, crop development, mid and late season stage were 0.32, 0.58, 0.63 and 0.39, respectively for SAMPEA7 and 0.32, 0.63, 0.72 and 0.34, respectively, for SAMPEA9 varieties. The highest crop water use efficiency (CWUE) and irrigation water use efficiency (IWUE) were 3.76 kg/ha-mm and 2.11 kg/ha-mm, respectively, for SAMPEA7 variety and 2.94 kg/ha-mm and 1.68 kg/ha-mm, respectively, for SAMPEA9 variety. The crop yield response factor (Ky) was found to be 1.10 for the BPM treatments and 1.22 for the NM treatments. In conclusion, SAMPEA7 variety irrigated at 75% WRETo with BPM gave the highest WUE of 2.11 kg/ha-mm which translates to highest yield per unit depth of water-use.

1.1      Background
Cowpea is one of the most widely adapted, versatile and nutritious of all the cultivated grain legumes (Dugje et al., 2009). With the increasing need of this crop, it is necessary to accelerate and expand its production all year round. This could mean making an effort to grow the crop under irrigation so as to have more than one cropping season in a year which would increase its production.

Nigeria has two distinct seasons – the rainy season, lasting from mid of March to the end of October, and the dry season, lasting from November to March. In the dry season, there is virtually no rain and irrigation remains the only option for crop production. There is stiff competition for water by the agricultural, domestic and industrial users during the dry season, hence there is the need for farmers to conserve and make judicious use of the available water (Adekalu and Okunade, 2006).

Studies are needed to increase the efficient use of the available water. Regulated deficit irrigation with mulching is one among many practices that is fast gaining ground, and it appears a very promising option at achieving the goal of more crops per unit volume of water, if properly adopted. The development of new irrigation scheduling techniques such as deficit irrigation and identifying the sensitive crop growth stage to water stress is one way to enhance crop productivity with less water (Bekele and Tilahun, 2007). Regulated deficit irrigation scheduling practice is a technique of withholding or skipping irrigation, or reducing the amount of water applied per irrigation at some stages of the growth with the aim of saving water, labor, and energy without adverse effect on yield
performance. This practice leads to some degree of moisture stress on the crop and likely reduction in crop yield (Smith et al., 2002: Prichard et al., 2004; Zhang et al., 2004).

The objective of regulated deficit irrigation is to save water by subjecting crops to periods of moisture stress with minimal effects on the yields. The water stress results in less evapotranspiration by closure of the stomata, reduced assimilation of carbon, and decreased biomass production. The reduced biomass production has little effect on ultimate yields where the crop is able to compensate in terms of reproductive capacity (Stone et al., 2001).

Mulching involves the use of materials to cover the cropped soil surface with the aim of reducing evaporation, conserving soil moisture, modifying soil temperature, structure and improving aeration. It also suppresses weeds and reduces erosion. (Hassan,1996 Plauborg et al., 1996). A better understanding of crop yield-water application interaction and the effects of irrigation levels and mulch (covering the soil surface) on the relationship between transpiration, evaporation and evapotranspiration for various crop is necessary to improve the management of irrigation water resources (Kadayifci et al., 2004).

Furthermore, evaporation and transpiration occur simultaneously and there is no easy way of distinguishing between the two processes directly on the field. The amount of water required to compensate the evapotranspiration loss from the cropped field is defined as the crop water requirement. Although the values for evapotranspiration and crop water requirements are identical, crop water requirement refers to the amount of water that needs to be supplied, while evapotranspiration refers to the amount of water that is lost (Richard et al., 1998). The water balance method provides a simple but robust
means of continuous measurement of evapotranspiration (ET) from different species of vegetation (Granier et al., 1990; Gholipoor, 2007).

The Institute for Agricultural Research (IAR), Samaru has released several varieties of cowpeas in the last 15 years. Although some of these varieties are planted under irrigation, the irrigation water requirements and their crop coefficient (Kc) values have not been systematically evaluated. A systematic evaluation of these parameters involves an isolation of the crop growing medium, which is the soil, so that water input and output into and out of the system will be effectively monitored. By so doing, the soil water balance components will be appropriately determined. One device that can be used for such study is the soil lysimeter.

1.2       Statement of the Problem
Despite the potentials and importance of cowpea, there is little, if any documented research information on cowpea production under irrigation in Nigeria. Production practices of a crop under irrigation must necessarily be stated independently from the rain-fed crop because the two seasons are not comparable. Most of the agronomic research to date on cowpea has been focused on the rain-fed crop by the Institute for Agricultural Research (IAR), Samaru Zaria (Mohammad, 2011).

It is an established fact that one of the important factors limiting the development of agriculture in the semi-arid region is water. Samaru which is located within this region is one of agricultural areas in Zaria, Kaduna State. However, there are large potentially irrigatable lands which are not irrigated due to insufficient or lack of irrigation water, which led to the necessity of optimizing and managing the use of water for agricultural production (Oiganji, 2010). There is also uncertainty as to the availability of water in adequate amounts and times, especially where storage facilities such as dams are absent.

This necessitates development of water management strategy to avert water stress in crops; which if proven advantageous, could assist farmers as most of them are at subsistence level and they are economically weak with low ability to withstand risks.

With the recent screening and release of improved cowpea varieties that are high yield photo-insensitive and early maturing, nine varieties of cowpea for different ecologies have been developed and released for production by IAR. The most popular are SAMPEA6 and SAMPEA7 which are resistant to many stress factors, and SAMPEA9 which is dual purpose (high grain and fodder yields) (Agricultural Research Council of Nigeria, 2017). The study therefore applied the use of a weighing-type soil micro-lysimeter to study the yield and water balance of two varieties of cowpea (SAMPEA7 and SAMPEA9), under mulch and deficit irrigation.

1.3       Justification of the Study
In Nigeria, the greatest production of cowpea comes from the northern region. The north produces about 1.7 million tonnes from 4 million hectares. This represents over 60% of total production (FAO, 2005). During the dry season, water is usually provided from both irrigation facilities and residual moisture of wetlands (Inaizumi et al., 1999). But water is a limiting factor in the expansion of irrigated areas and in the production of food. As the population increases, greater competition for the water supply makes conservation and efficient use of water imperative (Eric et al., 1981).

Rising cost of Irrigation pumping, low commodity prices, inadequate irrigation system capacities and limited irrigation water supplies are the reasons for which many irrigators deliberately apply less water than is required for maximum yield. (Craciun and Craciun, 1999). One means of determining when irrigation should be supplied is through the use of a soil water balance or soil water budget (Tim, 1996). Cowpeas are exposed to
varying levels of environmentally induced stresses during their growth stages, and limited information exists on reliable estimates of evapotranspiration (ETo) to be used for forecasts, to achieve high irrigation water use efficiency in semi-arid environment, particularly under micro-irrigation (MI) system, such as in IAR Research Farm at Samaru Zaria.

Improved irrigation management requires adequate information on the nature and degree of responses of various growth stages to water stress. Hence, with the changes that have taken place in the global climates in the last one decade, and with new varieties of crops released, there is the need to re-validate the existing body of information on crop water requirements, crop coefficients and the water stress coefficients. The reason for this is because these parameters depend on climatic factors and crop characteristics. With these background considerations, a comprehensive field investigation was undertaken to study the water balance components of two IAR cowpea varieties under Irrigation.

1.4       Aim and Objectives of the Study
The aim of this study was to evaluate the yield and water balance of two IAR cowpea
varieties   (SAMPEA7   and   SAMPEA9)   under   mulch   and   deficit   irrigation   using
weighing- type micro-lysimeters.

The objectives are;

a)                  To determine grain yield and crop water use of the cowpea varieties under different water application regimes and mulch practice.

b)                  To develop crop coefficient curve for the cowpea varieties under limited and unlimited water supply conditions.

c)                  To express the yields and water use relationship for the cowpea varieties.

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