More concentrated precipitation decreases terrestrial water storage | Nature

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Subjects

- Climate change

- Hydrology

Abstract

Terrestrial water availability is a key determinant of human and ecosystem well-being1,2. Apart from mean precipitation and evaporation changes3,4, it is unknown how daily-scale precipitation concentration into fewer, heavier events affects hydrologic partitioning and the land water balance5,6,7,8,9. Here we show observationally that more concentrated precipitation decreases land water availability across all climates globally, a drying effect as strong in magnitude as the wetting effect of increased total precipitation. Simple and complex land-surface models recover the observed effect, whereas idealized simulations show that it arises from enhanced evaporation caused by hydrologic partitioning changes at the land surface. Projected terrestrial water storage impacts of warming-driven precipitation concentration at about 2 °C of warming shift the land surface to abnormally dry conditions (≥0.5 standard deviation10) for 27% of the global population, independent of any total precipitation or irrigation changes. Our results show new key determinants of the land water balance, highlighting its sensitivity to the temporal distribution of precipitation, with broad implications for future water availability.

Main

The land water balance is essential for the well-being of ecosystems and people1,2. Yet, its future is uncertain because of sparse observations and a lack of theory unifying climate and hydrologic changes11,12. Although water availability emerges from the balance of mean precipitation and evaporation, a key uncertainty resides with how warming-forced changes to the character and distribution of daily precipitation will alter the long-term land water balance. The right-skewed daily precipitation distribution means a minority of days contribute most to annual precipitation13. At the same time, the highest percentiles of this distribution are over three times more sensitive to warming than annual mean precipitation itself (>7% K−1 compared with about 2% K−1)5,14. The asymmetry in the precipitation distribution and its unequal response to warming implies a concentration of rain and snow into fewer, more intense and localized events, separated by longer dry intervals, with ambiguous implications for terrestrial water availability5,6,7,8,9,15,16.

Debates over future aridity tend to focus on the climatological precipitation–evaporation balance3,4, whereas research on the impacts of precipitation intensification generally centre on flood risks17,18. Yet precipitation concentration could exert an important control on the climatological land water balance by reshaping hydrologic partitioning of rain and snow into rivers, soils and vegetation. Saturation and infiltration excess runoff on the land, for example, can be controlled by the duration and intensity of rainfall, whereas the dry intervals between rain events articulate the energetic space for evapotranspiration. Experimental treatments in a US grassland, for example, found reduced soil moisture in response to more concentrated watering19, whereas daily rainfall variability affected satellite vegetation indices over 42% of vegetated lands20. But the extent, spatial pattern and magnitude of the impact of precipitation concentration on climatological water availability, both historically and in future climates, remain unknown.

We analyse the observational relationship between satellite-derived annual-scale terrestrial water storage (TWS) anomalies and precipitation concentration estimated by a Gini coefficient with multiple precipitation datasets21,22,23. The annual Gini coefficient of daily precipitation quantifies how evenly precipitation falls over a year. Our empirical models demonstrate that precipitation concentration reduces continental water availability across all climates globally. The magnitude of the drying effect of concentration on TWS variability rivals the effect of total annual precipitation itself, suggesting a first-order control of precipitation concentration on the land water balance, beyond the long-term balance between precipitation and evapotranspiration.

Using a simple land-surface model that captures the nonlinear behaviour among energy and moisture, we then reproduce our empirical results and show that precipitation concentration enhances evaporation through changes in surface hydrologic partitioning and increased shortwave radiation between precipitation events—consistent with first-principles of land–atmosphere interactions. These observed and idealized results are also consilient with the response in more complex land-surface components of Earth system models. Finally, we combine our empirical model with observed and projected concentration trends to estimate historical and future TWS impacts. Wherever possible, we rely on observations given known deficiencies in daily precipitation simulation by global atmospheric models. Our findings indicate that the utility of