Tackling water table depth modeling via machine learning: From proxy observations to verifiability

TL;DR

This study develops high-resolution machine learning models to accurately predict water table depth across North America, outperforming physical models, and discusses future improvements for large-scale hydrogeological simulations.

Contribution

The paper introduces three ML models constrained by physical relations, trained on extensive real and proxy data, demonstrating improved accuracy over existing physical models for water table depth prediction.

Findings

ML models achieve higher correlation (0.6-0.75) than physical models (0.21-0.40) in predicting WTD.

Models trained with combined real and proxy observations improve prediction accuracy.

Large-scale WTD simulations remain uncertain in data-scarce mountainous regions.

Abstract

Spatial patterns of water table depth (WTD) play a crucial role in shaping ecological resilience, hydrological connectivity, and human-centric systems. Generally, a large-scale (e.g., continental or global) continuous map of static WTD can be simulated using either physically-based (PB) or machine learning-based (ML) models. We construct three fine-resolution (500 m) ML simulations of WTD, using the XGBoost algorithm and more than 20 million real and proxy observations of WTD, across the United States and Canada. The three ML models were constrained using known physical relations between WTD's drivers and WTD and were trained by sequentially adding real and proxy observations of WTD. Through an extensive (pixel-by-pixel) evaluation across the study region and within ten major ecoregions of North America, we demonstrate that our models (corr=0.6-0.75) can more accurately predict unseen real and proxy observations of WTD compared to two available PB simulations of WTD (corr=0.21-0.40). However, we still argue that currently-available large-scale simulations of static WTD could be uncertain within data-scarce regions such as steep mountainous regions. We reason that biased observational data mainly collected from low-elevation floodplains and the over-flexibility of available models can negatively affect the verifiability of large-scale simulations of WTD. Ultimately, we thoroughly discuss future directions that may help hydrogeologists decide how to improve machine learning-based WTD estimations. In particular, we advocate for the use of proxy satellite data, the incorporation of physical laws, the implementation of better model verification standards, the development of novel globally-available emergent indices, and the collection of more reliable observations.