Abstract
Whether or not reservoirs contain water throughout the dry season is critical to avoiding late season crop failure in seasonally-arid agricultural landscapes. Locations, volumes, and temporal dynamics, particularly of small (<1 Mm3) reservoirs are poorly documented globally, thus making it difficult to identify geographic and intra-annual gaps in reservoir water availability. Yet, small reservoirs are the most vulnerable to drying out and often service the poorest of farmers. Using the transboundary Volta River Basin (~413,000 sq km) in West Africa as a case study, we present a novel method to map reservoirs and quantify the uncertainty of Landsat derived reservoir area estimates, which can be readily applied anywhere in the globe. We applied our method to compare the accuracy of reservoir areas that are derived from the Global Surface Water Monthly Water History (GSW) dataset to those that are derived when surface water is classified on Landsat 8 OLI imagery using the Normalised Difference Water Index (NDWI), Modified NDWI with band 6 (MNDWI1), and Modified NDWI with band 7 (MNDWI2). We quantified how the areal accuracies of reservoir size estimates vary with the water classification method, reservoir properties, and environmental context, and assessed the options and limitations of using uncertain reservoir area estimates to monitor reservoir dynamics in an agricultural context. Results show that reservoir area estimates that are derived from the GSW data are 19% less accurate for our study site than MNDWI1 derived estimates, for a sample of 272 reservoir extents of 0.09 to 72 ha. The accuracy of Landsat-derived estimates improves with reservoir size and perimeter-area ratio, while accuracy may decline as surface vegetation increases. We show that GSW derived reservoir area estimates can provide an upper limit for current reservoir capacity and seasonal dynamics of larger reservoirs. Data gaps and uncertainties make GSW derived reservoir extents unsuitable for monitoring reservoirs that are smaller than 5.1 ha (holding ~49,759 m3), which constitute 674 (56%) reservoirs in the Volta basin, or monitoring seasonal fluctuations of most small reservoirs, limiting its utility for agricultural planning. This study is one of the first to test the utility and limitations of the newly available GSW dataset and provides guidance on the conditions under which this, and other Landsat-based surface water maps, can be reliably used to monitor reservoir resources.
Highlights
Freshwater scarcity is a major constraint to food production in agricultural regions of the world with variable intra- and inter-annual rainfall patterns and poor water storage infrastructures [1]
Using the Volta Basin in West Africa as a case study, we focus on three specific questions: (i) What is the accuracy of reservoir areal extents digitized manually from high resolution Google Earth imagery compared to those derived computationally from Global Surface Water Monthly Water History (GSW) or from commonly used spectral water indices applied to Landsat 8 Operational Land Imager (OLI) imagery? (ii) How does the accuracy of Landsat-based reservoir area estimates vary with environmental factors? (iii) What information on reservoir size and seasonality can be reliably determined from the GSW and what cannot? Our study represents the first attempt to test the limits of the GSW dataset for monitoring reservoirs of varied size and across a range of environmental conditions in a West African context
We used one of the GSW derivative layers, the GSW Maximum Extent (GSW-MX), to get the maximum extent for the remaining 83 reservoirs whose maxima derived from GSW were ≥(92 ha—root mean square error (RMSE) of area estimates), and cannot be measured using our Google Earth Engine (GEE) approach, which is restricted to extents of less than 1024 pixels (~92 ha)
Summary
Freshwater scarcity is a major constraint to food production in agricultural regions of the world with variable intra- and inter-annual rainfall patterns and poor water storage infrastructures [1]. Small and large reservoirs are a common development investment to avert or reduce water shortages and boost production in seasonally dry, agriculture-dependent regions [4]. Information on reservoir locations, volumes, and seasonality (water presence-absence) can guide policies to allocate and manage reservoir water sustainably and avert agricultural water and associated food shortages. Access to this information would support donors, governments, and NGOs in efforts to better understand the value of reservoirs and target reservoir investments and maintenance to achieve local to global sustainable development objectives
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
