Abstract
Accurate quantification of the water balance, in particular evapotranspiration, is fundamental in managing water resources, especially in semi-arid areas. The objective of this study was to compare evaporation from endemic vegetation – Renosterveld – and a dryland wheat/fallow cropping system. The study was carried out in the mid-reaches of the Berg River catchment (South Africa), characterised by dryland salinity. Measurements of total evaporation from these 2 land uses were carried out with large aperture scintillometers during window periods from 2005 to 2007. Total evaporation was measured to be higher in Renosterveld than in wheat during the rainy winter season. In the dry summer season, total evaporation from Renosterveld was limited by soil water supply, and vegetation was under water stress. Spatial variability of total evaporation from both wheat/fallow land and Renosterveld was estimated using the Surface Energy Balance Algorithm for Land (SEBAL) model for 3 climatically different years. The scintillometer measurements were used to determine basal crop coefficients for long-term (20 years) simulations with the HYDRUS-1D model to assess temporal variability in total evaporation. Long-term simulations indicated that well-established, deep-rooted Renosterveld uses 39% more water than the shallowrooted wheat/fallow system. A change in land use from Renosterveld to dryland annual crops could therefore affect the soil water balance, cause shallow saline groundwater tables and degradation of soil and water resources.Keywords: evapotranspiration; large aperture scintillometer; Renosterveld; soil water balance; Surface Energy Balance Algorithm for Land (SEBAL); wheat/fallow system
Highlights
Accurate quantification of the water cycle is a fundamental problem in hydrology and water management (Everson, 2001)
In mid- to end of summer (e.g. February 2007), ET’s from Renosterveld were much lower than ETo’s, showing that this surface had total evaporation below reference rates and the vegetation was under water stress
Evaporation from Renosterveld estimated with Surface Energy Balance Algorithm for Land (SEBAL) (MODIS) exceeding that from wheat/fallow land by, on average, 39%
Summary
Accurate quantification of the water cycle is a fundamental problem in hydrology and water management (Everson, 2001). Amongst the water balance components (Poncea and Shetty, 1995), evaporation from the Earth’s surface and vegetation is often the most difficult to measure or estimate This is important in arid and semi-arid areas where annual rainfall is far below potential evaporation. In a cycle of projects funded by the Water Research Commission (Fey and De Clercq, 2004; Gorgens and De Clercq, 2005; De Clercq et al, 2010), it was found that changes in land use from indigenous, endemic vegetation, i.e. Renosterveld, to extensive pastoral use and dryland wheat farming over the last century or more may have changed the water balance and enhanced the mobilisation of naturally-occurring salts (originated from oceanic deposition through the ages) and salinisation of the Berg River
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