Numerical Analysis of Water Flow in Root Zone Using Meteorological Data.

Evaluating water contents in the root zone is important for water management of agricultural fields. Potential evapotranspiration rates from vegetated soils can be calculated from meteorological data using either the Penman-Monteith combination equation or Hargreaves equation. Since actual evaporation from soils and transpiration from crops are additionally also affected by water contents in the root zone, their evaluation requires simultaneous calculations of water flow and root water uptake in soils. In this study, we implemented Penman-Monteith and Hargreaves equations into the HYDRUS-1D software package. The code allows various combinations of meteorological (radiation, air temperature, and relative humidity, etc) and crop data (crop height, root depth, and LAI) to calculate potential evapotranspiration rates. Field measurements were carried out in soy bean field at National Agriculture Research Center in Chikugo, Japan during July to November of 2005. Soil water pressure heads at different depth (10, 40, and 55 cm) were measured by tensiometers. Evaporation rates, transpiration rate, root water uptake, and soil water pressure heads were numerically simulated using weather data (daily air temperature, relative humidity, wind speed, solar radiation and precipitation) collected from the weather station in the research center. The model is evaluated by comparing the measured and simulated water pressure heads.