Two Points on the Detail-Parsimony Tradeoff Curve for Modeling Hillslope Hydrology in an Earth-System Model

Abstract:

Earth-System Modeling often requires the modeler to address the tradeoff between detail and parsimony. Here we describe two points on the tradeoff curve that have been implemented for description of hillslope hydrology in the Geophysical Fluid Dynamics Laboratory (GFDL) Earth-System Model (ESM). These are (1) parameterized and (2) explicitly resolved descriptions of heterogeneity with respect to topographic position in the land models LM3 and LM3-TiHy (for "tiled hydrology"), respectively. The conceptual base for both descriptions is that of a characteristic landscape unit having width and elevation that are functions (only) of horizontal distance from a discharge point (a stream). Horizontal saturated hydraulic conductivity decays exponentially from a surface value to a (generally non-zero) value at depth.

In the parameterized treatment, the relations among hillslope water storage, seepage-face (i.e., saturated-surface) area fraction, and groundwater discharge are derived by solution of the steady-state form of the relevant hydraulic (i.e., one-dimensional) groundwater flow equation; a dimensionless form of the problem is pre-solved numerically for ranges of relevant parameters and is stored in a lookup table. In the explicitly resolved treatment, non-steady state is allowed, horizontal discretization is introduced, and variation of hydraulic head with vertical position is also considered. The explicitly resolved treatment allows for co-evolution of hydrologic and biogeochemical heterogeneity with respect to hillslope position, while the parameterized treatment is computationally faster. The two treatments yield essentially the same water- and energy- balance partitioning in experiments within the GFDL ESM, but only the explicitly resolved treatment (LM3-TiHy) provides the hydrologic framework for modeling biogeochemical processes that depend strongly on soil wetness (e.g., those associated with soil-carbon dynamics).