Modelling the effect of land use and crop changes on water harvesting agricultural reservoirs

Water harvesting with Small Agricultural Reservoirs (SmAR) represents a solution for sustainable water management at a global scale. Early estimates showed that globally there were about 277,400,000 SmAR with an area of less than one hectare, and 24,120,000 water bodies between one and 10 hectares, representing more than 90% of the world’s standing water bodies. One of the most relevant challenges for the sustainable management of SmAR is represented by the loss of storage volume caused by the inflow of sediments. However, the analysis of the dynamics of sedimentation for SmAR received relatively little interest so far in the Mediterranean and on a global scale.

The purpose of this study is to implement a fully calibrated and validated model simulating the hydrology and erosion dynamics of the catchment of a SmAR in the Tuscany Region (Italy). The area is in the hilly area of Crete Senesi, about 15 km from Siena, where wine production is particularly developed, but not within the catchment of study, where the cultivation of cereals, renewal crops, and forage is practiced and there is a large grazing area. Our analysis aimed at estimating how much the rate of sediment accumulation in the reservoir would vary with the replacement of currently arable land with vineyards.

A model was implemented on the HEC-HMS software, maximizing the value of existent low-cost data (Google Earth imagery and regional erosion maps) for its validation, despite its use at a very small scale for a SmAR in a single farm. The validated model was then used for testing alternative land use scenarios in the upstream catchment, showing its flexibility for supporting decision-making over SmAR management.

The model performed with an error always below 10% on the SmAR area detected by satellite and a Nash-Sutcliffe efficiency of 0.675. Erosion values calculated with HEC-HMS were in line with the estimation made by the Tuscany region with a GIS-based procedure. The results of scenario analysis showed that the simulated land use change led to a high value of annual sediment accumulation in the reservoir (216% of the original value of erosion obtained with cereals and other crops). Such information should be considered at least at the agronomic design stage, as well as in the estimation phase of the costs of water supply, which must include the cost of the reservoir volume restoration after sediment accumulation. The approach can be replicated at the local scale in all other contexts where similar, and relatively easy-to-get, data are available.

 

This research was carried out within the AG-WaMED project, funded by the Partnership for Research and Innovation in the Mediterranean Area Programme (PRIMA), an Art.185 initiative supported and funded under Horizon 2020, the European Union’s Framework Programme for Research and Innovation, Grant Agreement Number No. [Italy: 391 del 20/10/2022, Egypt: 45878, Tunisia: 0005874-004-18-2022-3, Greece: ΓΓP21-0474657, Spain: PCI2022-132929]

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