Lateral Flow and Contributing Area Control Vegetation Cover in a Semiarid Environment

Abstract

Semiarid ecosystems attract attention, because they support the life of large human populations, while functioning under growing threats of degradation due to global climate change that may lead to reduced productivity and possibly irreversible desertification. Empirical support is consequently highly sought after to validate the soil‐moisture‐biomass relationship at the local scale in these ecosystems. Combining physically based models and a database of 32 years of field and remotely sensed data, we examined the effect of soil‐moisture content (SMC) on vegetation cover and patch size distribution in the heterogenous semiarid Lehavim LTER in the Negev Desert, Israel. Results from numerical solutions of flow equations in the shallow soil layer allowed predictions of distributions of SMC as the outcome of vertical and lateral water fluxes. The association of vertical fluxes with patch size distribution and vegetation cover did not support the notion that direct rainfall is sufficient for the shrubs' survival in drought conditions. However, re‐infiltration of downslope runoff, generated in sealed bare intershrub soil area, significantly contributed to the water available to supporting shrub patches. Accordingly, our approach determines potential vegetation cover in semiarid areas, due to the dependency on contributing area and rainfall‐infiltration‐runoff processes. This underscores the fact that, additionally to infiltration from direct rainfall, run‐on contribution controls vegetation cover in semiarid environments. We therefore suggest that estimations of shrub resilience to water stress should consider scenarios involving surface sealing, rainfall duration and intensity, and contributing area size—each in accordance with the contribution of lateral flows to the water available to shrubs.

Hydrologic Connectivity and Patch‐To‐Hillslope Scale Relations in Dryland Ecosystems

“…The study results are also relevant to monitoring strategies, as relations between runoff run‐on path lengths and water balance partitioning enables the use of hillslope‐scale observations to constrain within‐slope processes, and vice versa. For example, total runoff observed at the outlet could be used to estimate source‐sink path lengths $$({\tilde{L}}_{\mathit{infl}})$$, as a proxy for the typical bare soil area contributing run‐on to vegetation patches (Svoray et al., 2021). For such an application, further work is needed to predict how source‐sink path lengths relate to the hillslope runoff coefficient across storm and hillslope conditions.…”

Unique Relationship Between Rate and Cumulative Flow: A Property of Infiltration and Evaporation in Soils

“…The dynamics of infiltration determines the evolution of the water content distribution with depth in the soil and the formation of overland flow and runoff (Assouline et al., 2007; Parlange & Smith, 1976). In natural environments, infiltration and runon have been shown to control vegetation cover in arid and semi‐arid regions (Assouline et al., 2015; Svoray et al., 2021). In the particular case, where the wetting rate is very high or when the soil surface is flooded, the infiltration rate is at its maximum and decreases exponentially with time from the very beginning of the wetting process.…”