New Level-3 datasets demonstrate that robust field-level groundwater use accounting is possible at continental scales: what is next for transboundary aquifer governance?

According to the UN Valuing Water Initiative, accurate measurement of water resources is critical to valuation, decision making, and governance. Rotary drilling and submersible pump technologies, which proliferated in the 1950s-1970s, facilitated rapid and widespread development of global groundwater reserves. Currently, 25% of global water use is from groundwater, trending towards 100% in arid regions, and 75% of this groundwater is used for agriculture. Because it is difficult to measure, regulation of groundwater use is sparse and underenforced, and international treaties governing use of groundwater are virtually non-existent. Given the promise of spatial ubiquity in satellite observations, the hydrologic remote sensing research community has made tremendous progress in the measurement of hydrologic fluxes that are aliased by sparse in-situ networks. Two promising data derivatives—mainly energy-balance actual evapotranspiration derived from radiometric surface temperature and surface displacement associated with groundwater extraction from interferometric synthetic aperture radar—have fundamentally changed our understanding of how anthropogenic groundwater use in particular is modifying the hydrosphere, and enable estimates of relative groundwater extraction rates at the well/farm scale. Due to the high computational costs and technical complexity associated with processing these datasets, particularly at the spatial scales required for national to transboundary water accounting, their use in operational water management has been limited.

Two operational datasets published in the United States this year allow for both large and small-scale accounting of agricultural groundwater use: the OpenET project DisALEXI dataset, and the OPERA project’s Sentinel 1 interferometric LOS displacement dataset. We demonstrate how independent error sources in these two datasets assist with uncertainty characterization, and benchmark their performance against GRACE observations of total water storage flux. We demonstrate how they allow us to estimate both regional (aquifer to nation-level) and local (field and well-level) groundwater use. We focus our analysis on 34 aquifers that span the United States/Mexico border, where a deepening water crisis is playing out in the absence of international agreements on transboundary aquifer use. We use this case study to demonstrate how investment in production of Level-3 datasets from entire satellite archives can enable international collaboration on natural resource development.