O impacto da desertificação na hidrologia: um estudo de modelagem para a bacia do rio Seridó

Abstract

Land use in the Seridó river basin is the main factor in the intensification of desertification in this region, where desertification interferes with the hydrology of the basin. Among its effects on the soil are the low quality and the change in the availability of water resources. The objective of this study was to evaluate the impacts of desertification on the hydrology of the Seridó river basin via hydrological modeling. The hydrological model used was the SWAT, which proved to be an effective tool, as it satisfactorily simulated the seasonality of the monthly flow in four tributary sub-basins of the Seridó river, simulating the observed flow variance by more than 60%. In the evaluation of the model, it was observed greater skill in flows equal to or less than Q5% (maximum flows) and the PBIAS (RMSE) of the simulated flow was ~ 30% (8,3 m³.s-1) and 50% (11,8 m³.s-1) for the model calibration and validation periods, respectively. To assess desertification, two land use and land cover scenarios were developed for the watershed: the control scenario, where the vegetation cover of the basin was that observed in 1985; and the desertification scenario, where desert areas were included from 1985 to 2019. The historic and gradual advance of the desert (+15,39%) altered the annual water balance, increasing the total runoff (𝑄𝑡𝑜𝑡) by 6,95%, due to the increase in surface runoff (𝑄𝑠𝑢𝑟𝑓), which increased by 12,08%. This change altered the Q5% regime, associated with floods, which were increased in the desertification scenario. In average absolute terms, the changes in 𝑄𝑡𝑜𝑡 and 𝑄𝑠𝑢𝑟𝑓 were +14 mm.month-1 and +11 mm.month-1 in the wet season, respectively. On the other hand, the subsurface runoff (𝑄𝑙𝑎𝑡) decreased by 15,34% in relation to the annual balance, although it increased by 1,5 mm.month-1 in the rainy season. The annual evapotranspiration (𝐸𝑇) reduced by 4,18%, which corresponded to -5 (-2) mm.month-1 in the rainy season (dry). The same impact occurred with soil water content (𝑆𝑊) and percolation (𝑊𝑝𝑒𝑟𝑐), in which the average annual reductions were 3,79% and 1,96%, -45 mm.month-1 and -4 mm.month-1 in the rainy season, in order. In spatial terms, 𝐸𝑇 , 𝑆𝑊 and 𝑊𝑝𝑒𝑟𝑐 ( 𝑄𝑡𝑜𝑡 , 𝑄𝑠𝑢𝑟𝑓 and 𝑄𝑙𝑎𝑡) were significantly reduced (expanded) in the central-west region of the basin. In general, found that desertification increases the extremes of Q5% and deteriorates the hydrological conditions, locally and in the areas adjacent to the desert regions, with the reduction of 𝑆𝑊 and 𝑊𝑝𝑒𝑟𝑐, and the increase of 𝑄𝑠𝑢𝑟𝑓. Such effects are expected, as desertification reduces infiltration due to the degree of compaction of the bare soil, unbalancing the groundwater system. Therefore, the maintenance of natural and agricultural ecosystems will be harmed.