Modelling the water balance of a small catchment: a case study of Muhu catchment in Southern highlands of Tanzania
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Date
2000
Authors
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Journal ISSN
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Publisher
Sokoine University of Agriculture
Abstract
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.
Description
aster of Science Agricultural Engineering
Keywords
Water balance- small catchment, Muhu catchment, Southern highlands