Assessment of groundwater resource vulnerability to over-exploitation in a tropical, agricultural basin

In many parts of the world, groundwater extraction for agriculture is strongly increasing, causing severe stress on groundwater resources and associated ecosystems. Understanding how groundwater flow systems support extractions is therefore essential. However, particularly in developing, rural, tropical regions, monitoring of groundwater levels, chemistry and extractions is poorly regulated, resulting in a lack of data. Hence, alternative approaches are necessary to develop best management practices in these groundwater basins. In this study, catchment-scale groundwater extraction is indirectly estimated by two “soft data” approaches: (1) using local knowledge through a qualitative field survey of groundwater level fluctuations and groundwater withdrawals; and (2) land-use/population data combined with local knowledge on cropping/water use practices. Spatially distributed recharge is simulated on the basis of a monthly water balance model, which requires widely available topographic, soil, land-use and meteorological data. Extractions and recharge force a simple, basin-scale groundwater model for assessment of impact of irrigation practices. Agricultural scenarios are developed and modelling procedures are designed to test the temporal and spatial vulnerability over a 100 yr time span of the groundwater resource. The approaches are tested and applied for the agricultural La Vi River basin, Vietnam, where the livelihood of the local farmers requires development of new agricultural and hydrological techniques. The typical cash-crops are cultivated on sandy soils and irrigated in the dry season from thousands of private shallow wells. The tropical climate and strong seasonal rainfall pattern produces a strong fluctuation in groundwater levels. The modelling shows significant spatio-temporal unmet pumping demand dependent on the agricultural development scenario, indicating the need and opportunity for planning of groundwater based irrigation development. Overall, the multi-method comprehensive approach supports basin-scale sustainable groundwater resource development and only requires relatively easily accessible data.