Comparison of Effects of Root Water Uptake Functions for Simulating Surface Water and Heat Fluxes within a Corn Farmland Ecosystem in Northeast China

The root water uptake (RWU) process plays an important role in the terrestrial water cycle by connecting the soil environment to the atmosphere. Inaccurate representation of RWU functions (RWUFs) in land surface models (LSMs) has been one of the principal causes of error. Various RWUFs have been proposed for application in LSMs to simulate surface water-heat fluxes within diverse ecosystems. However, none have proven suitable for maize farmland ecosystems where vegetation differs from perennial plants, maize being a typical annual crop with significant human intervention. This study evaluated the performance of the common land model (CoLM) to reproduce energy and water-vapor fluxes during the growing season measured with an eddy covariance system in a maize farmland ecosystem of northeastern China. The simulation performance of the default CoLM differed with the interannual diversity of precipitation: it was relatively poor in the low rainfall year with underestimation of latent heat flux and overestimation of sensible heat flux, but was excellent in the high rainfall year. The effects of two different RWUFs were compared: RWUFZ, an empirical approach to represent a dynamic RWU efficiency by introducing two thresholds; and RWUFL, modifying the original RWUF with a linear function of soil water potential to one with an exponential function. The CoLM performance for simulating water and heat exchanges was significantly improved when the default RWUF was replaced with RWUFL and RWUFZ, respectively, using reasonable parameter assignments. The improvements of modified model performance demonstrated obvious interannual and intermonthly variations, which were more significant for RWUFZ than RWUFL. Thus, RWUFZ is recommended for use in the CoLM in northeast China's maize farmland ecosystems and possibly for other similar agriculture ecosystems. (C) 2017 American Society of Civil Engineers.