Unraveling the causal influences of drought and crop production on groundwater levels across the contiguous United States

TL;DR

This study explores how drought and crop production affect groundwater levels across the U.S., finding that crop production has a significant and lasting impact.

Contribution

The paper introduces a causality-based framework to quantify the relative influence of drought and crop production on groundwater levels at a continental scale.

Findings

Groundwater levels decreased with increases or no change in crop production across the U.S.

Crop production significantly influenced groundwater levels in 32% of counties, more than drought's impact.

Cotton and wheat production had the most pronounced regional effects on groundwater.

Abstract

Groundwater depletion in agricultural-dominated regions is attributed to climate and irrigation withdrawals that support crop production. However, despite decades of effort, knowledge gaps remain in understanding the relative influence of drought and crop production on groundwater levels at the continental scale. Here, utilizing empirical observations, we simultaneously track how long-term trajectories of groundwater levels, crop production of seven crops, and drought have evolved over time, and then integrate these observations with a causality-based attribution framework to unravel the relative impact of drought and crop production on groundwater levels across the contiguous United States (CONUS). We find a dominant pattern of decreases in groundwater levels with increases (25–61%) or no change (1–15%) in crop production across the CONUS. We estimate a significant (P < 0.1) causal influence of crop production and drought on groundwater levels in ∼32% (n = 101) and ∼20% (n = 62) of counties, respectively. Further, the extent of impact of crop production on groundwater varies with region and is most pronounced for cotton (42%, n = 18) and wheat (17%, n = 39). The memory effects of crop production (median: 7 years) and drought (median: 3 years) on groundwater levels imply that their impact could last much longer than the annual crop production cycle or the drought exposure period. Further, these findings allude to circular causality between groundwater and crop production, where both entities depend on each other at different time scales. Our work builds on past work and contributes to the growing understanding of food security and groundwater availability to manage these commodities to meet future demands.

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