Quantification of Runoff for 2007 Land use Scenarios in the Maychew
Watershed, Tigray, Northern Ethiopia
Nata Tadesse*, K. Bheemalingeswara and Teshale Berhe
Department of Applied Geology, Faculty of Science and Technology, Mekelle
University, Mekelle, Ethiopia [P.O. Box 1604, E-mail: tafesse24603@yahoo.com]
ABSTRACT
This paper quantifies the amount of monthly and annually generated runoff from the
different land units of a Maychew watershed, which has an areal coverage of 96 km2.
Digital topographic, slope, soil, and land use information was used to estimate potential
runoff contributing areas. Meteorological data have been collected from Maychew
station. Rainfall coefficient method was used to determine the monthly distribution of
rainfall in Maychew watershed. FAO-Penmn Monteith method and Thornthwaite water
balance model were used for determination of the potential evapotranspiration and actual
evapotranspiration, respectively. For runoff estimation, the methods employed was runoff
coefficient.
The mean annual rainfall of the basin is 813.5 mm. The basin is characterized by two
rainy seasons and three dry seasons during the year. The two rainy seasons and the three
dry seasons have seven and five months, respectively. The total amount of rains that
comes during the rainy months is 708.6 mm. Small rains occur in the two months (March
and October) of rainy seasons and the amount is 107.50 mm. Big rains occur in the
remaining five months (April, May, July, August and September) of the rainy season with
moderate, high and very high concentration and the total amount is 601.10 mm: 218.7
mm occurs with moderate concentration in April, May and September, 155 mm with high
concentration in July and 227.4 mm with very high concentration in August. The rain in
the dry months is 104.86 mm. The mean annual actual evapotranspiration of the basin is
688.30 mm. The mean annual runoff is 13.27 million cubic meters. Out of these amount
87.13 % is generated from rainy months whereas 12.87 % from dry months. Among the
rainy months, 73.89 % of the runoff is coming from the big rainy months and 13.24 %
from small rainy months. The big rains that occur with high and very high concentration
in July and August produced 19.06 % and 27.99 % the mean annual runoff of the basin,
respectively.
In the basin there are no any inland waters. The only available surface water is the runoff
that is generated by the different land units of the area. This huge amount of water, which
are lost annually, can be harvested and stored for efficient use when the rains stops, and
distribute it wisely for different users.
Keywords: Dry season, evapotranspiration, rainfall coefficient, rainy season, runoff.