Uso de método geofísico na caracterização de pequenas bacias hidrográficas sob eucalipto e campo

Abstract

The demand for forest products has gradually increased in recent years. Thus, eucalyptus has emerged as essential source of raw material for several global economy sectors, mainly due to its rapid growth and biomass incorporation in the short-term. However, natural resources using in the production cycle of this forest species require managing it properly. Thus, finding the balance between production and environmental preservation it necessary. The aim of the current study is to characterize the hydrology of the underground environment of three small watersheds associated with eucalyptus and grassland cultivation in the Campanha region, Rio Grande do Sul State, Brazil. The aforementioned watersheds comprise 79.89 hectares of Eucalyptus saligna (EsB), 82.09 hectares of Eucalyptus benthamii (EbB), and 109.36 hectares of degraded anthropized natural grassland (GB) used for livestock grazing. Hydrological and hydrogeological variables in the investigated areas were monitored in an independent way, but rainfall data from GB was used for the period from 01/01/2019 to 06/15/2019. Surface soil samples were collected for granulometric analysis; sand content was determined through sieving, whereas clay and silt contents were determined based on the pipetting method. In addition, the underground recharge of monitoring wells (two monitoring wells in each watershed) was estimated based on the Water Table Fluctuation method, by taking into consideration the period of 197 days before each geophysical test. Therefore, water balance at watershed scale was also analyzed in the same period determined for water recharge, based on rainfall and water flow values, as well as on estimates of soil water evaporation, plant transpiration and of the evaporation of water intercepted by plants. Water balance in the soil profile was calculated for all months from 2019 to 06/15/2020. The geophysical method based on electrical resistivity tomography was used to characterize the underground environment on 11/19/2019 and 6/4/2020, in nine transects, three in each watershed. Water infiltration in the soil was measured with Cornell infiltrometer. The geophysical method enabled determining differences in geometries between the analyzed periods. Water recharge was greater in watersheds under eucalyptus cultivation than in GB. Rainfall values recorded for the 197 days before the first and second measurements were 1,098.5 mm and 478.5 mm, respectively. Interception reached 248.4 mm, 356.1 mm and 149.4 mm, whereas evapotranspiration reached 624.3 mm, 512.6 mm and 299.5 mm in EbB, EsB and GB, respectively, in the period (197 days) before the first geophysical test. Rainfall interception by canopy recorded period before the second rainfall measurement reached 108.3 mm, 154.9 mm and 65.1 mm, whereas evapotranspiration reached 347.6 mm, 257.0 mm and 328.7 mm in EbB, EsB and GB, respectively. Granulometric analysis showed significant values for sand, which categorized the soil as sandy loam or sandy-clayey loam. Based on infiltration tests GB can present greater surface runoff than the other watersheds, due to the influence of plant typology in each watershed. Based on the water balance analysis, there was decreased water availability in the interval between the two geophysical tests. However, the water balance in the soil profile of the second test showed water surplus due to high rainfall rates (199.9 mm) in the previous month. Thus, the geophysical technique can be used to characterize the underground environment in forest plantations. Furthermore, the current results were associated with the hydrological variables that preceded the field tests and that are influenced by land use and cover.