On the structure of soil moisture time series in the context of land surface models

Root-zone soil moisture is addressed as a key variable controlling surface water and energy balances. Particular focus is applied to the soil moisture controls on wet-end drainage and dry-end transpiration, and the integrated effects of these controls on the structure of soil moisture time series. Analysis is centered on data collected during a pair of field experiments, where a site in Virginia (USA) provides evidence of dynamics under dry conditions and a site in Cork (Ireland) captures dynamics under wet conditions. It is demonstrated that drainage processes (controlled by the saturated hydraulic conductivity) determine the magnitude of soil moisture at the start of the drying process and hence affect uniformly the entire distribution of soil moisture, from wet to dry. Therefore, stationary bias between predicted and measured soil moisture can be evidence of a bias in the saturated conductivity. In contrast to this, the dry-end soil controls on transpiration affect predominantly the dry-end of the soil moisture distribution, as subsequent storms act to reset the system and remove the memory of the dry state. Hence, analysis of departure between predicted and measured soil moisture that is local to the dry-end can guide estimation of the soil moisture level at which transpiration becomes limited by water availability. The temporal statistics of soil moisture are shown to exhibit threshold response to the specification of saturated conductivity in land surface models. Finally, we demonstrate the relative influences of saturated conductivity and precipitation intensity on the structural features of the root-zone soil moisture distribution.