Cowpea (Vigna unguiculata, (L.)Walp.) was subjected to water application deficit through a semi-portable sprinkler irrigation system. The experiment was conducted under three levels of water application deficit which were: fully irrigation (with 100% of soil moisture content at field capacity), 80%, and 60% of soil moisture content at field capacity. For all levels of water deficit, the highest percentage of water application depth (31.02%), occurred during mid - season stage. The higher crop evapotranspiration (Etc) value was (6.94 mm. d-1) observed after 35 days from planting with fully irrigation. At 100% of soil moisture content at field capacity, the average value of crop coefficient (Kc) was 0.696, 0.615, 0.673 and 0.60 for initial, development, mid – season, and harvesting stage respectively. The average values of water stress coefficient (Ks) in development and harvesting stages illustrated that the crop extract from root zone without suffering water stress in the readily available water because of the root zone depletion was smaller than the radial available water in root zone. The highest seed yield (1.12 Mg/feddan) was observed with fully irrigation, while the lowest (0.67 Mg/feddan) was with 60% of field capacity. This lowest value of seed yield was associated with low number of pods/plant (14.6 pods /plant) and small increase in number of seeds per pod (11.00 seeds/pod) and average seed weight (20 g/plant). Increasing the deficit percent of water application resulted in progressively lower water use efficiency. At 80 % of field capacity, water use efficiency was 0.68 kg/m3 while, it decreased to 0.59 kg/m3 as the deficit percent increased from 80% to 60% of soil moisture content at field capacity.