Abstract
This paper analyses the effect of the spatial assessment scale on irrigation performance indicators in small and medium-scale agriculture. Three performance indicators—adequacy (i.e., sufficiency of water use to meet the crop water requirement), equity (i.e., fairness of irrigation distribution), and productivity (i.e., unit of physical crop production/yield per unit water consumption)—are evaluated in five irrigation schemes for three spatial resolutions—250 m, 100 m, and 30 m. Each scheme has varying plot sizes and distributions, with average plot sizes ranging from 0.2 ha to 13 ha. The datasets are derived from the United Nations Food and Agricultural Organization (FAO) water productivity through open access of remotely sensed–derived data (the Water Productivity Open Access Portal—WaPOR) database. Irrigation indicators performed differently in different aspects; for adequacy, all three resolutions show similar spatial trends for relative evapotranspiration (ET) across levels for all years. However, the estimation of relative ET is often higher at higher resolution. In terms of equity, all resolutions show similar inter-annual trends in the coefficient of variation (CV); higher resolutions usually have a higher CV of the annual evapotranspiration and interception (ETIa) while capturing more spatial variability. For productivity, higher resolutions show lower crop water productivity (CWP) due to higher aboveground biomass productivity (AGBP) estimations in lower resolutions; they always have a higher CV of CWP. We find all resolutions of 250 m, 100 m, and 30 m suitable for inter-annual and inter-scheme assessments regardless of plot size. While each resolution shows consistent temporal trends, the magnitude of the trend in both space and time is smoothed by the 100 m and 250 m resolution datasets. This frequently results in substantial differences in the irrigation performance assessment criteria for inter-plot comparisons; therefore, 250 m and 100 m are not recommended for inter-plot comparison for all plot sizes, particularly small plots (<2 ha). Our findings highlight the importance of selecting the spatial resolution appropriate to scheme characteristics when undertaking irrigation performance assessment using remote sensing.
Highlights
Irrigation is typically performed in areas with arid climates, low precipitation, and/or frequent droughts, which makes water management both complex and important
Office du Niger (ODN) has the highest evapotranspiration and interception (ETIa), which is expected as it is dominated by rice
The ETIa has a difference of less than 6% between levels in the Wonji, the Metehara, and Zankalon; 5–9% in ODN; and 11–17% in the Koga
Summary
Irrigation is typically performed in areas with arid climates, low precipitation, and/or frequent droughts, which makes water management both complex and important. Irrigation consumes approximately 86% of freshwater withdrawals in Africa [1]. This is already higher than the global average and is expected to increase with increasing prosperity and therefor food demands. Quantifying water balance components and land productivity for irrigation schemes scales has a wide variety of applications. This includes but is not limited to initiating and evaluating water conservation practices, evaluating equitable water distribution [2,3,4,5], assessing water and land productivities [6,7], input to water policy and resource management [8,9], and improving irrigation management and systems [2,10]
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