Improvement of hydrologic simulations in CLM4 by modified soil properties

Abstract:

Runoff and soil moisture biases were found by comparing fully coupled CCSM4 simulations and observations. The CLM underestimated runoff in the areas where soils have high clay content, but overestimated in the areas covered by volcanic ash soils (i.e. Andisols). Clayey soils tend to exhibit aggregation structure that prone to form macropores. Macropores enable water to flow through unsaturated soil more rapidly than it would in a soil matrix defined by Darcy’s law. The existence of macropores increases effective hydraulic conductivity, thus decreases water content in the surface soils. Without this mechanism, CLM4 may overestimate evapotranspiration and in turn underestimate runoff by retaining too much plant available water. We hypothesize that lack of macropore flow mechanism is partially responsible for the underestimation and insufficient soil porosity representation is associated with overestimation. Andisols are soils formed in volcanic ash with very high porosity (often >0.60 cm³ cm^-3) and water holding capacity. The mineral soil porosity is defined by sand content in CLM and is much lower than it would have been for Andisols. CLM may retain insufficient plant available water and underestimate evapotranspiration therefore partitioning too much to runoff. We propose more detailed soil maps in the CLM to improve the representations of soil physical properties that are critical in the terrestrial water modeling.