Measurement and modelling of primary production and water use in a south Tunisian steppe

Measurements carried out between 1971 and 1977 indicate the effect of rainfall on above-ground plant production. The site investigated consists of steppeland on sandy soil, dominated by the perennial dwarf shrub Rhantherium suaveolens with associated annual species. The average precipitation in this region (180 mm), produces about 1000 kg dry matter/ha. The ARFEJ model described, uses the most pertinent characteristics of the system so that production can be predicted under the assumption that water is the limiting factor. Seven model coefficients were calculated with the data of 1971–75. The results show (a) the considerable effect of rainfall distribution on evaporation from bare soil. The mean of this evaporation was 55 per cent (range 40–69 per cent) of total evapotranspiration; (b) during the short annual period of fast growth, the ratio of dry matter to water transpired would be about 1·26 g dry matter/kg H2O for annuals, and 1·39 g dry matter/kg H2O for perennial species. Over the whole year, the average efficiency of water use is 0·45 g dry matter/kg H2O for all species; (c) above 250 mm precipitation, the availability of nitrogen and/or phosphorus becomes the limiting factor for plant production and efficiency of water use decreases.