Dependence of hillslope moisture content on downhill saturation

TL;DR

This study models how moisture content on hillslopes depends on downhill saturation, revealing how saturated zones influence flow and moisture pooling over large distances, with implications for understanding regional water flow.

Contribution

The paper introduces a theoretical model of downslope moisture flow based on steady equilibria of the Richards' Equation, highlighting the impact of saturated zones on flow dynamics and moisture distribution.

Findings

Moisture pooling distance exceeds 10 km in fully saturated zones.

Decreasing boundary saturation reduces pooling height by an order of magnitude.

Drying saturated zones explains observed drops in spring outflow in a semi-arid watershed.

Abstract

We derive steady equilibria for lateral downslope moisture flow in an idealized thin closed layer as a solution to the 1D Richards' Equation. The equilibria are determined by two free parameters: the downslope flux and a boundary condition. Solutions exhibit a constant downslope flow speed and moisture content for the constant equilibrium flux, which is the product of the two. However where an isolated zone of fixed saturation degree exists representing a boundary condition, the flow speed immediately upslope is reduced and the moisture content correspondingly increased to preserve the constant equilibrium flux. The capillary head jump at the saturated zone produces a blockage that gives a high moisture content back upslope through a pooling distance determined by the equilibrium condition that the downslope flux is constant. In our numerical integrations, the vertically projected pooling height is more than 10 km for a fully saturated zone in mixed silty or clay soils, but decreases by about an order of magnitude with every 10% decrease in the boundary-zone saturation degree. The drying of downhill saturated zones with the increased speed of mountain moisture outflow and corresponding decreased mountain moisture content gives a viable explanation for the mysterious ~69% unaccounted drop seen in the spring outflow in the La Luz / Fresnal Watershed at Alamogordo's upstream spring-box diversions in the semiarid southeastern New Mexico USA.