Groundwater extraction for irrigation of Coffea canephora in Ea Tul watershed, Vietnam-a risk evaluation

After the reunion of North and South Vietnam in 1975, the area under small-holder, irrigated cultivation of Coffea canephora increased by a factor of 10 in the Central Highlands. The actual acreage under coffee in Dak Lak province is estimated to be over 260,000 ha, representing 60% of the national production, with 405,000 t per year for the world market. Vietnam is presently the second largest exporter worldwide and expansion is still going on. In response to farmers' alarming reports of apparent water scarcity in the dry season, this paper assesses the groundwater recharge in the Ea Tul watershed, a representative area in the Central Highlands region. The groundwater recharge is estimated by means of a steady-state, water balance model and validated by means of historical data, stream gauge records at the basin's outlet and groundwater level fluctuations in wells nearby the study area. To account for the variability from one year to another, the input of the water balance equation was subjected to a frequency analysis. The estimates indicate that the expected annual groundwater recharge is 743 mm in a wet year, 517 mm in an average and 361 mm in a dry year. Validation and literature reviews confirm these figures. Subsequently the model is used to evaluate the issue of aquifer depletion and to verify whether a combination of optimal irrigation management and optimal land allocation for Robusta coffee could solve the apparent water shortages in the coffee sector. Allowing for a minimum river discharge under very dry circumstances (i.e. the Q90 of river base flow), the sustainable yield from the aquifer for variable climatic conditions is calculated. The results confirm that with the present irrigation application depths, the groundwater is likely to be depleted in dry years, but not in wet and average years. Reducing the irrigation application depths could keep the groundwater balance positive in all years. Optimization of land allocation for C. canephora may offer an alternative way out. A reduction by 35% of the present coffee area in Ea Tul along with irrigation efficiency improvement could keep the groundwater resources in balance and guarantees a minimum river discharge, even during severe dry seasons.