| Vol. 32 (2004) | |||
- Grundwasserneubildung im Einzugsgebiet
des Ouham, Zentralafrikanische Republik
[Groundwater recharge in the catchment area of Ouham, Central African Republic]
Borgstedt, Ariane (2004), 132 p., 61 Fig., 55 Tab.
22.- € Order
Abstract
Grundwasserneubildung im Einzugsgebiet des Ouham, Zentralafrikanische
Republik
[Groundwater recharge in the catchment area of Ouham, Central African
Republic]
Ariane Borgstedt
Abstract: The main object of this work was to determine the groundwater
recharge in the catchment area of the river Ouham, Central African Republic,
with the existing partially fragmentary data by using different methods.
The different approaches have been tested and evaluated for their applicability
in subtropical areas. The river Ouham flowing in the Northwest of the
Central African Republic is part of the catchment area of the river Chari
and drains into the Lake Chad Basin. The area under investigation is part
of the humid alternating tropics, therefore hot and dry winds coming from
the Northeast are predominant from November to March or April. In this
time surface runoff is taking place only in rivers of first order. From
Mai to October humid winds coming from the Southeast dominate the weather.
Surface runoff is formed not only in rivers of first order but everywhere
the morphology and the soil characteristics admits it. The surface runoff
is measured with the five staff gauges. The western part of catchment
area shows the highest specific discharge, while this decreases to the
east due to lower precipitation. The maximum rate of runoff is measured
in September in the western catchment area, in the eastern part of the
catchment areas this occurs in October. Precipitation is continuously
decreasing from Southwest to Northeast with the exception of the mountainous
region of “Bakoré”. Years with high of precipitation
were 1951, 1952, 1954 to 1955, 1957, 1960, 1962, 1963 and 1969, whereas
1972, 1973, 1977, 1982 to 1984 and 1986 to 1987 were years with low precipitation.
The area of investigation consists mainly of a proterozoic basement build
of granite and gneiss, which is overlaid in the Southwest with mesozoic
sandstones. The whole basement area is crossed by many faults running
mainly Northwest – Southeast or perpendicular to it. Within big areas
of faulting and intrusion the rock is highly fissured. The groundwater
recharge is calculated first with runoff data, secondly with the “hydrological
book-keeping equation” and than with a computer-based program called
MODBIL. The calculation based on runoff data was carried out with the
methods of WUNDT, KILLE and MAILLET. The results differ clearly. The maximum
groundwater recharge was calculated with WUNDT, the minimum recharge is
the one of MAILLET. For the catchment area of the river Ouham with a highly
pronounced topography in the West and heavy and batch-wise precipitation,
the groundwater recharge calculated with KILLE seems to be the most realistic
one. The groundwater recharge decreases with the decreasing available
water supply from West to East. In the West it amounts to 150 mm/year
in years with lots of precipitation, whereas in the East it only comes
to 79 mm/year. In years of poor of precipitation the calculated groundwater
recharge is 106 mm/year in the West and 64 mm/year in the East. The results
of the MAILLET-method show a recharge rate of 50 mm/year in the West and
of 26 mm/year in the East which is a minimum of the groundwater recharge.
The determination of the recharge with “hydrological book-keeping
equation” is done as a plausibility control of the results obtained
with runoff data. The recharge rates of the “hydrological book-keeping
equation” are clearly higher than the one calculated with the methods
of KILLE and MAILLET. With the computer-based program MODBIL point data
such as precipitation, temperature and evaporation are transferred into
space data using primary existing space data as topography, morphology,
land use and geology. The actual evaporation, the runoff and the groundwater
recharge is calculated on the basis of soil water household. The averages
of the effective precipitation, potential and actual evaporation show
a clear West-East-tendency. The highest effective precipitation is falling
in the westernmost part of the catchment area and decreases to the East.
Potential and actual evaporation increases from West to East, but this
tendency is only slightly visible for the actual evaporation. The average
runoff behaviour does not show any tendency. Contrary to this, the groundwater
recharge decreases clearly from West to East. The rates of recharge are
in the same dimension as the ones calculated with KILLE.
Paper language: german; german
abstract
22.- € Order