Land Surfaces at the Tipping‐Point for Water and Energy Balance Coupling. Issue 2 (22nd February 2023)

• Record Type: Journal Article
• Authors: Dong, Jianzhi; Akbar, Ruzbeh; Feldman, Andrew F.; Gianotti, Daniel Short; Entekhabi, Dara
• Is Part Of: Water resources research. Volume 59:Issue 2(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-02-22
• Subjects: soil moisture dry‐down -- land‐atmosphere coupling -- tipping point -- SMAP -- soil moisture; Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2022WR032472
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: The surface water and energy balances can be coupled or uncoupled depending on whether the evaporation regime is water‐limited or energy‐limited. As the landscape loses soil moisture during drydowns, a transition between the regimes may occur, which signifies a nonlinear change in water‐energy‐carbon coupling. Regions that switch often between these two regimes, that is, are dominated by neither regime, are particularly vulnerable to climate variability and change. To robustly identify these tipping points, we identify drydown events based on global soil moisture data sets from remote sensing. The event identification does not rely on precipitation information and is robust with respect to measurement noise. Then, the soil moisture thresholds delineating the evaporation regime transitions are determined by Sequential Monte Carlo Sampling and a two‐stage parametrization strategy. Based on the estimated soil moisture thresholds across the globe, we estimate observation‐based water availability indices which quantify the nonlinear controls of soil moisture on evaporation. This framework is tested and applied globally using Soil Moisture Active Passive soil moisture retrievals. Combined with a new tippling‐point metric that describes the frequency of evaporation regime transitions, we identify regions that switch often between different evaporation regimes at the global scale. Given unit shifts in soil moisture, these regions will experience the most change in how their surface water and energy are coupled. Plain Language Summary: Evaporation links the land surface water, energy, and carbon balances. Soil moisture can reduce the amount of available energy that is used for evaporation at the land surface. Evaporation can thus change from an energy‐limited regime to a water‐limited regime at a critical soil moisture value. Additionally, at some low soil moisture value, evaporation altogether diminishes to zero (wilting‐point soil moisture threshold). Besides evaporation, drainage can be the dominant hydrology regime of the landscape. This is the case for very wet soils (above so‐called field capacity or soil moisture threshold at which gravity is no longer an adequate force to dewater the porous media). There are thus several hydrologic regimes during interstorm period, separated by three threshold soil moisture values. We develop robust methods to estimate these thresholds using estimates of surface soil moisture from NASA's Soil Moisture Active Passive satellite mission. The regime transitions and thresholds make the hydrologic response of the landscape to climate forcing via a nonlinear process. We develop a new metric to globally map where the landscape water balance is at the tipping‐point between regimes. Landscapes that exist near the tipping point are particularly vulnerable to climate variability and change. Key Points: Remotely sensed soil moisture is used to estimate the thresholds between drainage‐dominated, energy‐ and water‐limited evaporation regimes The soil moisture thresholds defining the regime changes are mapped globally and shown to be related to climate and soil texture Regions at the tipping‐point between water‐ and energy‐limited evaporation are mapped across the globe