## 3 Results
### Hydrological Control  
#### 3.2.1 Groundwater Flow Regime  
Determination of the Phalombe District groundwater flow regime (Figure 5a) was based on rest water level data obtained in 2017 from 34 boreholes drilled between January and April (end of wet season) under the Sustainable Rural Water and Sanitation Infrastructure for Improved Health and Lvelihoods Project funded by the African Development Bank (AfDB). These comprised just over half the dataset supplemented with archive data recorded between 1998 and 2017. Head contours follow the general topography with groundwater flow towards Lake Chilwa, with locally westward drainage to the Phalombe River catchment. Steepest gradients occur in the uplands with gradients becoming shallower in areas covered by the high-level deposits and shallowest beneath the lake terrace deposits nearing Lake Chilwa. Groundwater velocities, proportional to gradients, would correspondingly decrease towards the lake for similar geological materials, i.e., comparable permeability and effective porosity. Increasing fines though in lower-elevation basin deposits may potentially lead to lower permeabilities closer to the lake, again reducing velocities.  
Figure 5: (**a**) Contoured potentiometric surface (water table) at 10 m intervals for the Phalombe District based on water level data mostly from 2017; (**b**) plot of water table versus ground elevation. The four outliers are excluded from the spatial data, but included in Figure S6, where the most marked outlier causes a central western margin flow-field sink anomaly.  
Transit (residence times) over a given travel distance may hence generally increase closer to the lake, particularly as groundwater becomes semi/confined by high-level and terrace deposits. A nominal groundwater transit time of approximately 2100 years may be calculated for groundwater flow over a head drop of 720 to 620 m and separation distance of 16 km corresponding to a central flow line towards the lake in Figure 5a for which a calculated hydraulic gradient of 0.0063 and groundwater velocity of 7.6 m/y applies, assuming a nominal hydraulic conductivity of 1 m/d and effective porosity of 0.3. More restricted groundwater transmission downstream though may provide baseflow emergence instead at higher elevation, perhaps perennial, stream reaches or/stream heads springs. Where travel distances within groundwater are short between recharge and discharge points, transit times in the aquifer may be considerable lower than calculated above with discharge potentially occurring within months to years of recharge dates. The detailed network of streams shown in Figure 5a is illustrative of the widespread emergence of streams under high rainfall run off-high water table conditions over much of WRA 2. Whilst hints of groundwater head curvature towards some upland streams suggest groundwater baseflow discharge, sparse groundwater table measurements near rivers largely prevents confirmation of groundwater-surface water influent/effluent conditions, especially in the mid-to-lower reaches. Rapid rises in lake levels under high rainfall could trigger some hydraulic gradient reversals and near-shoreline lake recharge of groundwater, however, near lake, high-resolution temporal data would be required to confirm that dynamic).