Root for rain: Towards understanding land-use change impacts on the water cycle

We live today on a human-dominated planet under unprecedented pressure on both land and water. The water cycle is intrinsically linked to vegetation and land use, and anticipating the consequences of simultaneous changes in land and water systems requires a thorough understanding of their interactions. This thesis aims to advance our knowledge of how land-use change influences the water cycle, i.e., focussing on the role of land-use in mediating water’s journey from land evaporation, to atmospheric moisture, and to precipitation on land. This thesis first presents the development (Chapter 2) and evaluation (Chapter 3) of the process-based water balance model STEAM (Simple Terrestrial Evaporation to AtmosphereModel). STEAM simulates five different evaporation fluxes, based on land-use representation with only a limited number of parameters. Comparison with independent data shows that STEAM produces realistic evaporative partitioning and hydrological fluxes over different locations, seasons and land-use types. Chapter 4 investigates the temporal characteristics of partitioned evaporation, and shows that terrestrial residence timescale of transpiration (days to months) is substantially longer than that of interception (hours). The vegetation’s ability to transpire by retaining and accessing soil moisture at great depth is critical for dry season evaporation, and the substantial differences in temporal characteristics between evaporation fluxes can create contrasting moisture recycling patterns. In response to the importance of root zone storage capacity for transpiration and moisture recycling simulation, Chapter 5 sets out to present an ’earth observation-based’ method for estimating this critical parameter in land surface modelling. By assuming that vegetation does not root deeper than necessary to bridge critical dry periods, satellitebased evaporation were used to derive root zone storage capacity. The new estimate improved evaporation simulation overall, and in particular during the least evaporating months in sub-humid to humid regions with moderate to high seasonality. The results suggest that several forest types are able to create a large storage to buffer for severe droughts, in contrast to e.g., grasslands and croplands. Based on the new insights, Chapter 6 analyses the effects of land-use change on river flows. In some of the world’s largest basins, precipitation was found to bemore influenced by extra-basin, than within-basin, land-use change. In fact, in several non-transboundary basins, river flows were considerably influenced by land-use changes in foreign countries, suggesting new transboundary water relationships in international politics. This thesis addressed different domains in the water cycle to improve our understanding of land-water interactions. Every water flux and stock requires our examination, whether they flow visibly in rivers, travel invisibly in the air, or hide deep in soil and roots. Because of the terrestrial water cycle’s interaction with land, and therefore human activities, we are in an extraordinary position to shape its path and pace.