Cropping Intensity in the Aral Sea Basin and Its Dependency from the Runoff Formation 2000–2012

Christopher Conrad,Heiko Paeth,Denis Sorokin,Fabian Löw,Sarah Schönbrodt-Stitt

doi:10.3390/rs8080630

Christopher Conrad, Heiko Paeth + Show 3 more

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https://doi.org/10.3390/rs8080630

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Abstract

This study is aimed at a better understanding of how upstream runoff formation affected the cropping intensity (CI: number of harvests) in the Aral Sea Basin (ASB) between 2000 and 2012. MODIS 250 m NDVI time series and knowledge-based pixel masking that included settlement layers and topography features enabled to map the irrigated cropland extent (iCE). Random forest models supported the classification of cropland vegetation phenology (CVP: winter/summer crops, double cropping, etc.). CI and the percentage of fallow cropland (PF) were derived from CVP. Spearman’s rho was selected for assessing the statistical relation of CI and PF to runoff formation in the Amu Darya and Syr Darya catchments per hydrological year. Validation in 12 reference sites using multi-annual Landsat-7 ETM+ images revealed an average overall accuracy of 0.85 for the iCE maps. MODIS maps overestimated that based on Landsat by an average factor of ~1.15 (MODIS iCE/Landsat iCE). Exceptional overestimations occurred in case of inaccurate settlement layers. The CVP and CI maps achieved overall accuracies of 0.91 and 0.96, respectively. The Amu Darya catchment disclosed significant positive (negative) relations between upstream runoff with CI (PF) and a high pressure on the river water resources in 2000–2012. Along the Syr Darya, reduced dependencies could be observed, which is potentially linked to the high number of water constructions in that catchment. Intensified double cropping after drought years occurred in Uzbekistan. However, a 10 km × 10 km grid of Spearman’s rho (CI and PF vs. upstream runoff) emphasized locations at different CI levels that are directly affected by runoff fluctuations in both river systems. The resulting maps may thus be supportive on the way to achieve long-term sustainability of crop production and to simultaneously protect the severely threatened environment in the ASB. The gained knowledge can be further used for investigating climatic impacts of irrigation in the region.

Highlights

In the Aral Sea Basin (ASB), excess use of natural river water resources for irrigation during the past decades has caused severe losses in the vital natural ecosystems such as riparian forests and wetlands in delta regions, and the desiccation of the terminal lake (e.g., [1])
With a distinctly lower overall accuracy (OA) of 0.70, modeled moderate resolution (MR)-irrigated cropland extent (iCE) for classification zones (CZs) 4 presents an outlier compared to the results for the further CZs
high-resolution irrigated cropland extent (HR-iCE); (ii) the HR-iCE aggregated to MODIS pixel size; and (iii) moderate-resolution irrigated cropland extent (MR-iCE) received for the reference sites, as well as area comparisons between (i) and (ii), and (ii) and (iii)

Summary

Introduction

In the Aral Sea Basin (ASB), excess use of natural river water resources for irrigation during the past decades has caused severe losses in the vital natural ecosystems such as riparian forests and wetlands in delta regions, and the desiccation of the terminal lake (e.g., [1]). Obvious upstream-downstream gradients of river water are reported to significantly affect crop production in the downstream oasis [2]. A complex system of socio-economic and political conditions (e.g., [2,8]) together with climate change is assumed to aggravate the pressure on land and water resources in the ASB, with effects on the water availability [3,9,10]. Against this background, solutions for the sustainable water use and management are urgently required

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