Abstract

Sharing simple ideas across a broad community of practitioners helps them to work together more effectively. For this reason, drought early warning systems spend a considerable effort on describing how hazards are detected and defined. Well-articulated definitions of drought provide a shared basis for collaboration, response planning, and impact mitigation. One very useful measure of agricultural drought stress has been the “Water Requirement Satisfaction Index” (WRSI). In this study, we develop a new, simpler metric of water requirement satisfaction, the Phenological Water Balance (PWB). We describe this metric, compare it to WRSI and yield statistics in a food-insecure region (east Africa), and show how it can be easily combined with analog-based rainfall forecasts to produce end-of-season estimates of growing season water deficits. In dry areas, the simpler PWB metric is very similar to the WRSI. In these regions, we show that the coupling between rainfall deficits and increased reference evapotranspiration amplifies the impacts of droughts. In wet areas, on the other hand, our new metric provides useful information about water excess—seasons that are so wet that they may not be conducive to good agricultural outcomes. Finally, we present a PWB-based forecast example, demonstrating how this framework can be easily used to translate assumptions about seasonal rainfall outcomes into predictions of growing season water deficits. Effective humanitarian relief efforts rely on early projections of these deficits to design and deploy appropriate targeted responses. At present, it is difficult to combine gridded satellite-gauge precipitation forecasts with climate forecasts. Our new metric helps overcome this obstacle. Future extensions could use the water requirement framework to contextualize other water supply indicators, like actual evapotranspiration values derived from satellite observations or hydrologic models.

Highlights

  • A central and successful tenet of the famine early warning community is the fact that impacts of water deficits are predictable—water deficits anticipate reductions in food access and availability

  • To move closer to impacts associated with crop and rangeland water deficits, we explore simple combinations of Growing Season Precipitation (GSP) and estimates of crop and rangeland Water Requirements (WR)

  • Building on the standard FEWS NET crop phenology framework (Figures 1A–C), we introduce a simple agropastoral Phenological Water Balance (PWB), which is a form of tailored Aridity Index that takes into account the total amount of Growing Season Precipitation (GSP) and crop Water Requirements (WR) based on standard Water Requirement Satisfaction Index” (WRSI) modeling practices

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Summary

Introduction

A central and successful tenet of the famine early warning community is the fact that impacts of water deficits are predictable—water deficits anticipate reductions in food access and availability. An important step forward in operational application of the WRSI was the combination of the WRSI formulation with gridded rainfall, reference evapotranspiration, and soil property data sets (Senay and Verdin, 2001, 2003; Verdin and Klaver, 2002). These models provide a widely used foundation for tracking agricultural shocks in food-insecure countries. We draw from some of the most useful aspects of the WRSI framework and consider how advancements in forecast weather and climate data can support new types of forward-looking crop and rangeland monitoring applications

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