This study investigated the impacts of climate
and land use changes on water resources in the Dano catchment combining
hydrological processes understanding, hydrological simulations and
climate and land use scenarios application. The catchment covers about
195 km2 and is located in the Southwest of Burkina Faso in
West Africa. The study area is characterized by an annual population
growth of about 3% over the past decades.
Based on intensive
field investigations on soil hydraulic properties, instrumentation and
monitoring of hydro-meteorological variables such as discharge, soil
moisture, groundwater level, precipitation, temperature etc. the
distributed and physically based hydrological simulation model WaSiM
was successfully calibrated and validated for the catchment. Achieved
model statistical quality measures (R2, NSE and KGE)
ranged between 0.6 and 0.9 for total discharge, soil moisture, and
groundwater level, indicating a good agreement between observed and
simulated variables.
Land use change assessment in the
catchment over the period of 1990-2013 exhibited a decrease of natural
and semi-natural vegetation at an annual rate of about 2%. Conversely,
cropland, and to a lesser extend urban areas, have increased. Land
conversion was attributed to population growth, changing in farming
practices and environmental conditions.
Four land use maps
were used to build land use scenarios corresponding to different levels
of land use change in the catchment. Application of the land use
scenarios to the calibrated and validated hydrological model indicated
that, compared to the land use status in 1990, the current situation
leads to an increase in total discharge of about 17% and a decrease of
actual evapotranspiration of about 5%. The results of simulations
further showed that the increase in total discharge is related to high
peak flow, suggesting an alteration of flood risk.
Following
field measurements that showed infiltration rates 1.2 times higher
under semi-natural vegetation compared to cropland, land use change
related effects on soil infiltration rate was integrated in the
modeling of LULC change impact assessment. Model results with a refined
soil (integrating this additional information) and a classic soil
indicated similar trends in water balance components as a result of
land use change. However slight differences of 0.5 to 20 mm/year in the
water balance component were noticed between the two soil
parameterization approaches. The integration of land use related effects
on soil properties was suggested to render LULC change scenarios more
plausible.
The projected climate change signal in the
catchment was analyzed using the representative concentration pathways
4.5 and 8.5 of six datasets of the COordinated Regional climate
Downscaling Experiment-project. Compared to the reference period of
1971-2000, the climate models ensemble consistently projects an
increased temperature of 0.1 to 2.6°C over the period 2021-2050.
However, an agreement was not found among climate models with regards
to precipitation change signal as projections for annual rainfall
ranged between -13 and +18%.
The application of the climate
models ensemble in WaSiM showed future discharge change signals very
similar to the projected precipitation change. Individual climate
models showed opposite annual discharge change signals ranging from -40
to +50 %. On average, the climate models ensemble suggested a 7 %
increase in annual discharge under RCP4.5 and a 2 % decrease under
RCP8.5. The analysis of the catchment sensitivity to precipitation and
temperature change indicated that discharge is more related to
precipitation than to temperature as the environmental system of the
catchment is water limited and not energy limited.