Modelling the water balance of a small catchment: a case study of Muhu catchment in Southern highlands of Tanzania
Sokoine University of Agriculture
I'he water balance ol'Muhu catchment located in the Southern Highlands of Tanzania in Iringa region was modelled by establishing the empirical relations that exist between storage parameters, rainfall parameters and runoff components. Storage parameters included soil moisture storage and interception. Rainfall parameters included rainfall amount, intensity, duration. throughfalL stemflow and evaporation. Runoff components included total runoff, direct runoff and base How. The catchment's physical and hydrological characteristics that affect these parameters were determined. 1 he assessment of hydrological and physical properties showed that the soils were predominantly sandy clay, having high organic matter content, with a moderately rapid hydraulic conductivity (Ks) of 4.2 cm/h and infiltration rale of 3.8 cm/ h. The bulk density was generally low with an average of 0.9 g/cnT for 0-15 cm depth: 1.1 Ig/cm5 for 15-30 cm depth and 1.30 g/cm’ for 30 - 45 cm depth. The catchment had a slope steepness of 35 % and a varying vegetal percentage cover of about 56 %. The 1997/98 waler year was exceptional with high rainfall (1934 mm) mainly due to the El-nino phenomenon. Sixty-seven percent of the rainfall received in the catchment penetrated the canopy to reach the forest floor as throughfalL On average 3.3 % of the rainfall reached the forest floor as stem flow' while 25.5% of the rainfall was intercepted by the canopy. ThroughfalL stemflow and interception were linearlyIll related to rainfall. Die regression coefficients of all the relationships were significantly different from zero al 1% level (fteO). With increasing percentage surface cover, interception increased while throughfall decreased. The storage capacity of the forest cover was estimated to be 0.7 mm. Il has been found in this study that stream flow and runoff have gradually been increasing since the 1994/95 season. However the rainfall trend docs not support this development. A consideration of runoff curve numbers showed that the observed trend was partly due to catchment degradation, farming activities in the area have gradually been substituting the forest with arable land, thus reducing surface cover. Records indicated that the lowest recorded daily mean How was 0.27 m'/s. while the highest was 1.6 m'/s. I he water balance was positive during the first five months of the wet season. The highest water balance was in April. During this period there was more recharge to the soil moisture and ground water storage. Water balance was negative in the remaining seven months of the water year, with the lowest in September. The developed direct runoff model and water balance model were found to be valid and useful in estimating the respective parameters in forested catchments of the southern highlands of Tanzania.
aster of Science Agricultural Engineering
Water balance- small catchment, Muhu catchment, Southern highlands