Evapotranspiration studies on Themeda triandra Forsk. under field conditions.

Abstract

Hydraulic non-floating lysimeters were used to determine the evapotranspiration (Et) of a Themeda triandra grass cover under field conditions. The highest evapotranspiration losses were recorded during December and January when the plants were in the reproductive phase, provided the soil moisture was not limiting. As much as 10, 22 mm water per day was lost during this period from grass-covered lysimeters. The evaporation (Eo) losses from the bare soil lysimeters were approximately 6, 7% of those from the grass covered lysimeters. The average weekly Et/Eo relationship was 0, 341, 0, 389 and 0, 334 for the 1972/73, 1973/74 and 1974/75 seasons respectively. The rainfall for the three seasons was 252, 2 mm, 986, 6 mm and 524, 2 mm respectively. The monthly Et/Eo relationship values for the 1973/1974 season can be considered to be the potential Et/Eo relationship of a T. triandra grass cover ;under non-limiting soil moisture conditions. A harmonic curve was fitted to the data with a computer programme. The water use of a T. triandra veld and the soil moisture content were determined with the aid of the harmonic curve, weather, soil data and a computer programme.

Keywords: botany; evaporation; evapotranspiration; grass; grass cover; grasses; hydraulic non-floating lysimeters; lysimeter; moisture content; rainfall; soil moisture; themeda triandra; water use