Hydrologie des petits bassins du Nordeste Bresilien semi-aride: typologie des bassins et transposition ecoulements annuels | Small watershed hydrology in semi-arid north-eastern Brazil: basin typology and transposition of annual runoff data

The direct evaluation of water resources of non-monitored basins using hydrological transposition methods is one of the present-day challenges for hydrologists. Therefore, an ambitious research program concerning the hydrology of a small watershed has been developed over the last 30 years by SUDENE and ORSTOM in Sertao, northeastern Brazil. In this way, data related to hydrological mechanisms were collected from 43 small watersheds scattered throughout an area of 1000000 km2 where the weakness and the irregularity of the water resources have caused several dramatic droughts. In a first step, two main pluviometric areas were identified. Inside each of them, and for each kind of watershed, a simple relation between rainfall and runoff annual means was adjusted. The 43 monitored basins were then grouped by using two different typologies, the first one relative to their hydrological regime, and the second to their physical characteristics. The hydrological regime's typology was established by using two simple indicators: L600, the average annual runoff corresponding to a 600 mm precipitation amount, and X10, the flood volume per 10 years. It is shown that the factor L600 is intrinsically related to the regularity of the hydrological regimes and that the basins situated on sedimentary sub-soils exhibit the lower flood rate X10. This first typology is then compared with the second one, on the basis of the watershed's physical characteristics. This comparison corroborates the consistency of the two typologies by the fact that the original clustering of basins is found again. As a result, soil and vegetation turn out to be the predominant factors. These results were then used to work out a simple water resource evaluation method which can be used for small watersheds where hydrological local information is lacking. This transposition method requires maps of annual isohyets, soils and a global estimation of three correction factors for anomalies of vegetation cover and the existence of small dams or any area that may retain runoff.