La organización de los medios en orden al fin

The rainy season lasts 4 to 5 five months and occurs from May to October. The rainfall is generated by the combined effects of the West African Monsoon, migration of the inter- tropical convergence zone and the Sea surface Temperature. The rain is variable in space and time and characterized by short and intensive events. At Tambarga the rain events occurred mainly early morning or late afternoon. The variability of the rain in this region is more pronounced at the yearly and seasonal scale. The maximum rainfall measured per tipping obtained by comparing all the raingages has been used to characterize the rain overall Tambarga's basin. Therefore the annual variability was observed between 2010 and 2012. Indeed in 2011 less than 700 mm of total rain was collected in the basin against 1317 mm (2010) and 1243 mm (2012) (Figure 3.4). The rain in 2011 was almost 60 % less compared to average.

Baseflow diurnal pattern

46 Figure 3.4: Rainfall yearly pattern at Tambarga (2010 to 2012)

3.4.1.2 Stream flow

The discharge is gauged at two locations in the upstream and downstream part of the basin. The river at Tambarga is seasonal and reacts fastest during rain events with hydrographs that last less than one hour due to quick overland flow. The maximum discharges observed in

2010 were (0.58 m3/s and 0. 52 m3/s) against (1 m3/s, 1.25 m3/s) in 2012 for respectively

upstream and downstream weir. Unfortunately for technical reasons any discharge were collected in 2011.

In general the discharge downstream is higher than the discharge upstream; by contrast, at the beginning of the wet season the contribution of upstream is higher than the downstream; see Aug 2010 and September 2012 (Figure 3.5). The stream flow at Tambarga is ephemeral and starts flowing sometime after the season onset.

Jan Feb Mar Apr May Jun0 Jul Aug Sep Oct Nov Dec

50 100

2010

Total Rain = 1317 mm

Jan Feb Mar Apr May Jun0 Jul Aug Sep Oct Nov Dec

50 100

R

a

in

at T

a

mbar

ga [mm]

2011

Jan Feb Mar Apr May Jun0 Jul Aug Sep Oct Nov Dec

50 100

2012

Baseflow diurnal pattern

47

Three phases can be distinguished for the flows in the river during the rainy season. In the early season, only a section (300m from the outlet) at the downstream basin generates flow in the river when the downstream shallow groundwater is filled. In the middle season, when the upstream shallow groundwater is filled, the flows start at the upstream section of the river over 500m from the upper weir. This upstream flow is infiltrated between the two weirs during some days; thereafter the flow becomes continuous over the entire river (2.8 km) when the entire water tables network is interconnected.

Figure 3.5: Discharge measured upstream and downstream

3.4.1.3 Groundwater

From 2009 to 2012 the groundwater data were recorded by hand twice a week. This monitoring time step gives good information at seasonal time scale but is not relevant for daily time scale (Figure 3.6, year 2009). Therefore in 2011, the measurements were refined, and two additional wells were dug in the riverbed and provided with automatic pressure sensors for monitoring the groundwater level. The water level data were collected at fine time resolution (5-min), the choice was made knowing that the time that the water takes to reach the groundwater is greater than 5 minutes.

Baseflow diurnal pattern

48 Figure 3.6: Groundwater pattern (2009-2012)

The groundwater patterns change according to the timescale. The change at Tambarga is more obvious at daily and seasonal scale. The groundwater follows a seasonal pattern with recharge and depletion respectively during the rainy season and dry season. The year 2011 was dry with minimal recharge and deeper decrease of the groundwater level over the basin (-12m for Well N°2). The recharge of the middle watertable takes less than two weeks, when the rain events are more frequent.

The rainfall is the unique source for groundwater recharge. The peaks of recharges are enhanced by the high frequency and intensity of the rainfall events. Looking in detail at the groundwater level (well N° 1, 2, 3, 4, 7, 8 and 9, Figure 3.6), some short term fluctuations appear. These fluctuations dependent mainly on two factors, short rainfall event and water extraction for human needs .Their amplitudes are function of the rainfall characteristics and the amount of water extracted.

Baseflow diurnal pattern

49 3.4.1.4 Diurnal Patterns

a Streamflow

By carefully scrutinizing the daily streamflow and evaporation, a typical diurnal pattern could be highlighted. The time scale of the measurements is crucial for being able to capture the diurnal pattern, meaning that the scale should be much finer than one day. According to (Cuevas et al. 2010), studying diurnally the streamflow has been possible since the advent of advanced monitoring sensors. Regarding the number and the type of sensors available in the market, the choice of the sensor and/or the type of correction used could generate errors mainly for low discharge(Cuevas et al. 2010).

At Tambarga, the observation of the streamflow during two years allowed us to find some daily cycle of streamflow during the dry period inside the rainy season. The streamflow time series in Figure 3.7 highlight the daily pattern of the flow measured at upstream and downstream weirs, 1.6 km away. The streamflow diurnal pattern is characterized by a maximum flow rate early morning followed by a decreasing of the flow during the daytime to achieve the minimum rate at around noon. A phase of recovering starts when the minimum flow is reached; therefore between noon and the sunset the discharge increases to achieve a maximum in the early morning the next day (Figure 3.7). The lowering of streamflow takes place together with the increase of some variables such as temperature, evaporation and the strength of the radiation while the upward trend of the flow occurs in parallel with the downward trend of these variables.

Figure 3.7: Streamflow time series, the blue line represents the discharge measured

Baseflow diurnal pattern

50

b Diurnal evaporation

The evaporation is computed at half-hour time scale which allows highlighting its daily cycle. The evaporation is almost null or even negative during the night while the positive evaporation sometimes observed is caused by the night time winds. The diurnal cycle of the evaporation trend is opposite to that of the streamflow. Thus the evaporation almost null during the night and starts increasing with the sunrise and achieves the maximum around noon (maximum radiation) and then decrease to the sunset while also the net radiation strength declines (Figure 3.8).

Figure 3.8: Evaporation diurnal pattern: four subplots of daily evaporation measured with

eddy covariance technic in the agricultural basin