Uncertainty in satellite-based irrigation monitoring can reduce policy effectiveness and lead to farmer losses

“You can’t manage what you don’t measure” is a widely repeated mantra in water policy circles. Yet, monitoring agricultural water use remains socially, politically and economically challenging worldwide. Irrigation abstractions in both the developed and developing world are frequently not metered. Where water meters are installed, issues of tampering, inadequate maintenance, and lax enforcement of reporting often limit effective management. Satellite-based water use estimation – using high-resolution data about plant and soil water fluxes derived from optical, thermal or radar imagery – offers a potential solution to fill critical gaps in irrigation monitoring. In recent years, various nonprofit, for-profit, government, and multilateral groups have worked to introduce remote-sensing based water use regulations. However, how much do we really know about the accuracy of these methods? What are the economic and policy implications of any errors in water use estimates? 

In a newly-published study in Water Resources Research, a team from the University of Manchester in the United Kingdom and the University of Nebraska in the United States collated evidence from over 40 peer-reviewed studies from around the world to assess the accuracy of several commonly applied satellite-based irrigation water use monitoring approaches. In reviewing the studies, the team found few that objectively validated water use estimates against observed irrigation records. The studies that did reported large discrepancies between measured farmers’ water use and estimated farmers’ water use in specific locations or seasons. The authors then demonstrated that these measurement errors can have serious implications for the implementation of water policies, negatively affecting both farm incomes and the ability of regulators to control the impacts of abstractions on the environment and other water users. 

These findings offer valuable input for water managers and policymakers seeking to target and enforce efforts to enhance sustainable agricultural water use. As Tim Foster, study lead author and senior lecturer at the University of Manchester, said, “There is a need to recognize that there is no technological quick fix to address the water use information gap.” Nick Brozović, study co-author and director of policy at the Daugherty Water for Food Global Institute, added, “Importantly, uncertainties in estimating water use, whether from remote sensing or from other technologies, may have much larger economic impacts on water users than simple water accounting frameworks suggest.” Satellite remote sensing can provide extremely detailed information about patterns of agricultural water use. However, where uncertainties regarding the data accuracy are high, alternative forms of monitoring – whether meters or other proxy data like crop types – may be more acceptable to users and offer a more efficient means of achieving water policy goals.

The study is freely available online at http://dx.doi.org/10.1029/2020WR028378. [The full citation is Foster, T., Mieno, T., & Brozovic, N. (2020). Satellite-based monitoring of irrigation water use: Assessing measurement errors and their implications for agricultural water management policy. Water Resources Research, 56, e2020WR028378. https://doi.org/10.1029/2020WR028378]