Estimating the contribution of groundwater to rootzone soil moisture

In the Huaibei Plain basin, China, soybean is a main crop. During the soybean growth period, rainfall can vary largely and depth to watertable can also vary largely. The amount of water supplied to the soybean rootzone by groundwater affects soybean growth and yield. Accurate simulation of groundwater contributions to soybean rootzone soil moisture (groundwater contribution) can be important for determining irrigation to and drainage from soybean fields. Based on field observations and local weather data of 2005, HYDRUS-1D was validated by comparing simulated and measured rootzone soil water contents. The validated model was used to estimate the daily groundwater contributions for three different soybean hydrological growing seasons, i.e., an average year (1997), a wet year (2005), and a dry year (2004) with soybean growth at its optimal state. The main results were: (1) seasonal groundwater contribution was 157 mm in the experimental field, and the estimated groundwater contributions were 158, 222, and 387 mm in the wet, average, and dry seasons, respectively; (2) the groundwater contribution was about 63% of the total seasonal transpiration in the experimental field, and those were about 142, 80, and 66% of the total seasonal transpiration in dry, average, and wet seasons, respectively.