Figure 1.
Insert shows with arrow the location of study area in eastern Africa. Map of the Northern Tanzanian Divergence Zone depicts the East African Rift System (EARS), containing Lake Natron (north), diverging around the Ngorongoro Volcanic Highland massif and splitting into two separate rift valleys (Lake Eyasi on west) and Lake Manyara (on east). Prevailing wind is from the east. Olduvai basin lies to the west of and in the rain shadow of Ngorongoro. (Map made by Sara Mana, http://www.geomapapp.org).
Figure 2.
Paleoenvironmental reconstruction of Olduvai and schematic hydrogeological conceptual model (modified from [28]).
Figure 3.
Gridded annual precipitation data (P) from the Global Precipitation Climatology Centre (GPCC) for grid square 2.75 deg S, 35.25 deg E in which Olduvai Gorge is situated.
A 10 year moving average is also plotted. Data was accessed on 26/4/13from: http://iridl.ldeo.columbia.edu/SOURCES/.WCRP/.GCOS/.GPCC/.FDP/.version6/.
Figure 4.
Simplified conceptual groundwater flow model for a sloping aquifer.
Figure 5.
Geometric parameter sensitivity of spring flow recession using equation (2).
Variation in recession rates is predominantly controlled by variations in the ratio of hydraulic conductivity to specific yield (k0/ne) and relatively insensitive to variations in the geometry, i.e. the spread around the baseline recession for the two k0/ne end members (k0/ne = 1 and k0/ne = 10) is much smaller than variation between the end members.
Figure 6.
Rate of groundwater discharge (q, modelled using Eq. 2) at Olduvai springs normalised to the rate of groundwater recharge (IB) for the case of a sudden cessation of groundwater recharge after a period of steady state.
Baseline geometry parameters have been used and hydraulic properties (k0/ne) were varied across the likely range. The vertical axis, q/(IB), is clipped at 0.1 to illustrate the range of timescales by which the discharge recedes to 10% of its original value.
Figure 7.
Modelled (a) amplitude ratio (AR) and (b) phase shift (PS) of the input groundwater recharge forcing relative to the spring discharge output on periods from 0.1 year to 100 000 years across a range of likely aquifer hydraulic properties (using Eq. 8–10).
The transition to lower amplitude ratios and higher phase lags for periods lower than 100 to 1000 years implies greater buffering of the climate signal and increased potential resilience of the spring discharge to climate variability on these timescales.
Table 1.
Range of literature values for volcanically derived aquifers used to parameterise the groundwater models.