WATER AVAILABILITY FOR ALFALFA CROP IN DIFFERENT ECOCLIMATIC REGIONS OF THE STATE OF RIO GRANDE DO SUL
Abstract
Although having the highest alfalfa production in Brazil, the state of Rio Grande do Sul has higly limiting climatic conditions for this important forrage crop. The occurrence of drought periods from the end of spring to the end of summer, can be considered the most important problem in this aspect, since the termal conditions for growth and biomass production by this crop are totally appropriated in that period. A comparative analysis of water availability for alfalfa was performed, taking finto account an homogeneous data base of an eleven years period (1980/90), collected in eleven weather stations representing the ecoclimatic regions of Rio Grande do Sul. The maximum evapotranspiration (ETm) of alfalfa was estimated and serial water balances were calculated, for ten days periods, adopting a soil water storage capacity of 75 mm. The annual average maximum evapotranspiration for alfalfa ranged from 1194 mm (Serra do Sudeste region) to 1832 mm (Baixo Vale do Uruguai region). The highest frequency of water deficits occurred during the summer season, which can be related to the highest atmospheric evaporative demand in that period. The annual amount of water deficit for alfalfa is over than 100 mm in all over the State. Litoral Norte and Planalto Superior — Serra do Nordeste regions showed the lowest water deficits and the highest ETr/ETm ratio, while the highest annual amount of water deficit (419 mm) and the lowest ETr/ETm ratio occurred in Baixo Vale do Uruguai region.
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