Estimating of surface and groundwater availability and demand from Lower Chambo River Basin (Ecuador), as a tool for water management

This work proposes a zoning method as a tool for water management and it is applied to lower Chambo River Basin, located at Chimborazo Province (Ecuador). In order to achieve this goal, it was necessary to estimate the availability of surface water as well as of groundwater in the basin. The amount of exploitable groundwater was computed using standard water balance methods and Darcy mass balance relation, estimating thus the aquifer recharge. The conceptual model for the groundwater circulation is based on physiochemical, chemical and isotopic analysis applied on samples collected from spring and wells. The potential surface water supply was estimated from a hydrological analysis using the reported flow rate of rivers in the basin, which shows consistency with the measured values of local precipitation. The water demand in the basin was estimated using the rate of urban water consummation in the two biggest cities in the basin (Riobamba and Guano), the reports of the concession agreements from the National Water Secretary (SENAGUA) and the minimum water necessary for farming the products that are grown in the basin from the FAO report (1991). It was possible to identify three multilayer aquifers, with an origin volcano-sedimentary (Llio-Guano, Riobamba and Yaruquies) composed by volcanic deposits from Chimborazo, Igualata, and El Altar. These aquifers are fed mainly by two mountain ranges; from the west (Chimborazo and Igualata) and from the east (El Altar). The recharge has also a secondary contribution from local precipitation. The Chambo river becomes the main discharge of the surface flow and groundwater. Hydrogeochemical and isotopic data (ƒÂ18O and ƒÂ2H) evidence the possibility of two sources for groundwater. Llio and Riobamba aquifers are composed of calcium-magnesium bicarbonate waters, from slightly acidic to neutral as well as with enriched ƒÂ18O and ƒÂ18H. The Yaruquies aquifer is constituted by waters from sodium-bicarbonate to calcium-magnesium. While the springs located in Cubijies are constituted by sulfate waters with more depleted of ƒÂ18O and ƒÂ2H values, indicating that there is influence of the volcanic activities from El Altar. From those results and from morphological and water availability characteristics, the study area was divided into eight zones. Our results show there exists an unequal relationship between management and water usage in the eight defined zones, prioritizing water usage for farming and cement industry to the detriment of urban and rural supply. In this sense, we propose a water management for the basin to short (one year), medium (five years) and long term (twenty years).