Effects of soil moisture stress on evapotranspiration partitioning

Crop canopy decreases with increased soil moisture stress. Therefore, the soil evaporation (E) during the growing season will be affected. Leaf area index (LAI) and ground cover (GC) of cowpeas (Vigna sinensis, L.) were measured during the growing season on a line source sprinkler irrigation system receiving different amounts of irrigation to develop a function to predict LAI under different soil moisture conditions based on estimated evapotranspiration (ET). Microlysimeters were used to estimate E by differential weighing while ET was estimated by a water balance technique. Transpiration (T) was estimated by the difference between ET and E. A different relationship between ET and potential ET (ET(p)) and relative available water in the root zone (S-w) was obtained at S-w smaller than 0.42. The value of ET/ET(p) is 2.4 times S-w. The relationships between E/ET and LAI and GC have been modified by incorporation of S-w at different soil moisture conditions with S-w smaller than 0.42. Furthermore, no improvement was observed in estimating E/ET using GC instead of LAI at wet irrigation levels. It was found that the seasonal E/ET at wet irrigation levels (about 0.52) was almost equal to that of intermediate irrigation levels. However, at the dry irrigation levels the seasonal E/ET (0.59) was higher than that of wet irrigation levels owing to a smaller rate of increase of LAI and/or GC at dry irrigation levels.