2025 Theses Doctoral

Interactions between soil moisture and precipitation in a changing world

Nielsen, Miriam

This dissertation explores the direct (irrigation) and indirect (climate change) impacts of anthropogenic activities on soil moisture and precipitation, including the two-way coupled interactions between the atmosphere and land surface. Together, these chapters provide an integrated view of the water cycle, complicated by the context of a warming climate. For the water cycle, climate change is not just a story of precipitation changes, but also a story about altered atmospheric evaporative demand—with links to drought onset and termination, soil moisture dynamics, and human water management.

Here we ask: how will soil moisture and precipitation respond to climate change? And how will changes in soil moisture from anthropogenic activities affect the earth system? Chapter 1 outlines the interactions between soil moisture, precipitation, and irrigation within the changing water cycle. Chapter 2 demonstrates that warming-driven increases in evaporative demand require greater rainfall to terminate droughts in many regions. Chapter 3 reveals how moisture supply and demand mechanisms drive surface drydown dynamics. Chapter 4 shows that irrigation can influence precipitation patterns across continental scales, demonstrating the far-reaching consequences of artificially altering the water cycle.

Anthropogenic climate change has already affected drought severity and risk across many regions and climate models project additional increases in drought risk with future warming. Historically, droughts are typically caused by periods of below-normal precipitation and terminated by average or above-normal precipitation. In many regions, however, soil moisture is projected to decrease primarily through warming-driven increases in evaporative demand, potentially affecting the ability of negative precipitation anomalies to cause drought and positive precipitation anomalies to terminate drought.

In Chapter 2, we use climate model simulations from Phase Six of the Coupled Model Intercomparison Project (CMIP6) to investigate how different levels of warming (1, 2, and 3°C) affect the influence of precipitation on soil moisture drought in the Mediterranean and Western North America regions. We demonstrate that the same monthly precipitation deficits (25th percentile relative to a preindustrial baseline) at a global warming level of 2°C increase the probability of both surface and rootzone soil moisture drought by 29 % in the Mediterranean and 32 % and 6 % in Western North America compared to the preindustrial baseline. Furthermore, the probability of a dry (25th percentile relative to a preindustrial baseline) surface soil moisture month given a high (75th percentile relative to a preindustrial baseline) precipitation month is 6 (Mediterranean) and 3 (Western North America) times more likely in a 2°C world compared to the preindustrial baseline. For these regions, warming will likely increase the risk of soil moisture drought during low precipitation periods while simultaneously reducing the efficacy of high precipitation periods to terminate droughts.

Soil moisture drydown is an important characteristic of surface hydrology, with strong relevance for how drought may change in a warming world. However, traditional methods of calculating soil drydown generally assume moisture is lost exponentially following a precipitation event but fail to account for continued moisture infusions from subsequent precipitation.

In Chapter 3, we introduce a new approach for assessing surface soil moisture drydown that allows us to determine the time it takes for an infusion of precipitation to leave the soil without a priori filtering of precipitation events. Our simple model provides a more flexible framework for analyzing the sensitivity of soil moisture dynamics to climate across different seasons and timeframes. Applying our approach to western North America, we characterize soil moisture drydown across several observation-derived datasets (MERRA-2, NLDAS-2 MOSAIC, NLDAS-2 NOAH), testing the sensitivity of drydown times to seasonality, precipitation, and the land surface model used.

We find that this simple model can provide coherent drydown estimates for both MERRA-2 and NLDAS-2 MOSAIC. In NLDAS-2 NOAH, however, drydown time is more poorly defined due to increased surface soil moisture and lower evapotranspiration. We use this model to estimate the observational uncertainty in drydown times and set new benchmarks against which climate models can be evaluated. The approach can enhance our understanding of drought initiation and termination, including how characteristics of soil moisture drought may change in a warming climate. Irrigation has long been both an essential tool in global agriculture and a significant anthropogenic land-use forcing of climate. However, while the effects of irrigation on surface energy partitioning and temperature are well established, the effects of this land use forcing on precipitation are more uncertain.

In Chapter 4, we use GISS ModelE, a state-of-the-art general circulation model, with irrigated and non-irrigated scenarios to explore how irrigation affects precipitation across monsoon and non-monsoon regions of North Africa, West Africa, Southwest Asia, and Southeast Asia. We find that irrigation increases precipitation across most non-monsoonal areas of tropical and subtropical Asia and North Africa and within monsoon regions outside the summer monsoon season. These increases in precipitation occur both locally, in irrigated grid cells, and also far outside the main geographic centers of irrigation.

However, monsoon season precipitation responses to irrigation are mixed, with large increases in non-irrigated West Africa; increases over the intensely irrigated area of Northwest India; declines over peninsular India; and modest declines in Southeast Asia. Precipitation responses outside of monsoon regions, or outside the main monsoon season, are largely explained through the effect of irrigation on moist static instability: readily available surface moisture in heavily irrigated regions increases surface moist static energy and that forcing is large enough to reduce free tropospheric temperatures, decreasing moist static energy aloft across the entire domain. By contrast, monsoon precipitation responses are driven by the effects of irrigation on large scale circulation and moisture flux convergence. Chapter 4 highlights, for the most intensively irrigated region of the world, the complexity of mechanisms involved in the seasonally and geographically diverse precipitation responses.

The evidence presented here demonstrates that human interventions through irrigation and greenhouse gas emissions create cascading effects throughout the global water cycle, requiring both mitigation of warming and adaptive management strategies that account for these hydrological dynamics.