Are cascade reservoir systems sustainable agroecosystems? A comparative assessment of efficiency, effectiveness and resource footprint in a Sri Lankan micro-cascade

Abstract

CONTEXTSri Lankan village tank cascades have been classified as a globally important agricultural heritage system (GIAHS) in 2018.Yet, a systematic benchmark of the performance and sustainability of these systems has not taken place to date. OBJECTIVEThis study assessed the efficiency, effectiveness and resource footprint of village tank cascade systems with a view towards assessing the resilience of the system as a key contributor to the food production in Sri Lanka, by comparing the above parameters in a selected cascade system at two time points. METHODSThe land uses at two time points, i.e. 1920 and 2020, were studied using historical, primary and secondary data. The study site was Mahakanumulla-Ulagalla village tank cascade located in the dry zone of Sri Lanka. Primary data were collected from the site including social, economic, hydrological and ecological parameters. Landscape mapping was conducted and eight parameters under efficiency, effectiveness and resource footprint were calculated and analyzed using collected data for the two time points to compare the cascade status. RESULTS AND CONCLUSIONSThe study finds that the structural balance of the cascade system in terms of the ratio between catchment forest, water surface and command area of 5:1:1 has changed to 1:2:1 over a 100 years' period, displaying a clear deterioration. From 1920 to 2020, the agricultural water productivity has increased from 0.65 kg per m3 to 0.81 kg per m3 in the upstream and in the downstream it has increased from 0.65 kg per m3 to 1.00 kg per m3. However, relative to the increase in land productivity (i.e. 2050 kgha−1 to 2962 kgha−1 in the upstream and 2050 kgha−1 to 3009 kgha−1 in the downstream) during the same period, the increase in water productivity has been low. The irrigation water demand has risen from 0.21 million m3 to 0.22 million m3 in the upstream and from 0.28 million m3 to 0.32 million m3 in the downstream. However, the ratio of cascade components has changed causing ecological imbalance in the system with the expansion of home gardens and farm lands. SIGNIFICANCEExcept for water and land productivity, this study reveals a negative trend in parameters of efficiency, effectiveness and resource footprint between the two time points of comparison, which emphasizes the need for restoring the sustainability. We consider the structural balance and ecological sustainability as key factors in managing and restoring comparable village tank cascade systems. In particular, we highlight that this historically sustainable system may not be able to function sustainably today.