Estimation of Sustainable Yield of Some Rivers in Trinidad

A scenic and mountainous area of ancient tropical rainforest in the Caribbean Island of Trinidad has considerable potential for eco-tourism. However, it does not have significant groundwater resources, and its ecologically sensitive nature precludes the construction of impoundment reservoirs. Therefore, the sustainable yield, i.e., 7-day 20-year low flow (7Q20) minus instream flow, of its rivers is one of the critical factors affecting the area’s carrying capacity for eco-tourism. Unfortunately, hydrologic data for the watersheds in this area are either unavailable or insufficient for independently estimating the aforementioned sustainable yield by the conventional methods. Therefore, a hydrologic similarity method was used to obtain 7Q20 of the ungauged rivers. Further, the instream flow was estimated, in the absence of any local criterion, by instream flow incremental methodology. This paper describes the application of these two methodologies and presents the results, which indicated that owing to the presence of a hydraulic threshold for the commencement of habitat stress, the sustainable yield should not exceed 20–30% of 7Q20. It is believed that this technical note would be of interest to practicing hydrologic engineers, whose work may sometimes relate to an area where rainfall and stream flow data are sparse, discontinuous, and nonconcurrent, and the criteria for instream flow are not yet developed.