Effects of land-cover change on streamflow : analysis of watershed simulations from around the world

Globally, more than 700 million people live in watersheds with a degraded ability to provide hydrologic ecosystem (hydro-eco) services, such as improved water yield and moderation of peak and dry-season river flows. Restoring pre-development land cover to provide hydro-eco services has become a major driver of conservation. This dissertation investigates the hydrologic response to land-cover change in high-resolution hydrologic models of 32 watersheds spanning six continents and diverse climate zones. The watershed site models are used to explore streamflow changes for scenarios of land-cover restoration to a pre-development state, as well as land-cover development to agricultural or urban conditions. The site models are used to simulate land-cover change of 10% of watershed area, which is larger than the area typically selected for hydro-eco service investment. On average the magnitude of annual average water yield, low flow, and high flow are reduced 1-3% under restoration. The magnitude is similar but the direction reversed for development to agricultural or urban conditions. Focusing on restoration, results show that groups of watersheds have similar streamflow response characteristics to land cover-change despite the limited response magnitude. The groups are identified using a graph-connectedness approach based on the sign correlation of changes in 26 streamflow index values. Watersheds fall into three clusters characterized by their hydrologic responses to land-cover change: 1) low-flow intensifying, 2) high-flow intensifying, and 3) high-flow enhancing. Non-parametric hypothesis testing is used to identify which particular streamflow indices, watershed features, and pre-restoration flow features are characteristic of watersheds in the respective clusters. Under restoration, sites in the low-flow intensifying cluster exhibit an increase in low flow and a decrease in high flow. Sites in the high-flow intensifying cluster show high flow increases (relative to each site's median flow) and low flow decreases. The high-flow enhancing cluster behaves like the high-flow intensifying cluster, but has smaller reductions in low flows. Soil characteristics in the sites are distinct across clusters. Watersheds with smaller soil depths and available soil water content fall into the low-flow intensifying cluster, and these soil characteristics are statistically different from those of sites in the high-flow enhancing and intensifying clusters. After restoration, the low-flow intensifying watersheds typically exhibit greater infiltration and shift water to the low-flow regime. This flowvi regulating behavior is a desired effect of watershed restoration. In contrast, high-flow intensifying watersheds exhibit increased transpiration that reduces low flow. Such behavior is detrimental to regions with limited water availability. This study uses local hydrologic site models in a broad range of conditions to better understand how landcover changes affect hydrologic response and under what conditions such changes might enhance hydro-eco services. Results contribute useful guidance about the generally limited effects of land-cover change on water yield. This work also provides useful understanding about key watershed characteristics that affect streamflow responses to restoration.

Keywords: Earth System Science.
Advisor: Gorelick Steven M. 
Daily Gretchen C.
Freyberg David L.