## 3 Results
### Hydrological Control
#### 3.2.5 Stable Isotope Insights  
Basin-scale stable isotope surface water (river) signatures plot close to the Global Meteoric Water Line (GMWL) defined by the relationship of \(\delta^{2}\)H = \(86^{18}\)O + 10 [56]. (Figure 7 seasonal data). The isotopic signatures are consistent with the closest available local precipitation spatial variation, we have recently reported across the Shire River Basin that are defined by the shown Local Meteoric Water Line of \(\delta^{2}\)H = 7.9 (\(\pm\)0.1) \(\delta^{18}\)O + 11.3 (\(\pm\)0.8) [47]. All surface-water samples plot along meteoric water lines (EIV-LMWL; GMWL) and within the prediction intervals during the wet season, providing strong evidence of non-evaporated precipitative recharge input into surface-water bodies.  
During the dry season all surface water samples plot along meteoric water lines and within the prediction intervals except for Lake Chilwa that shows significant enrichment due to evaporative fractionation of lake water (Figure 7). The lake water shows progressive evaporative enrichment along a Local Evaporation Line (LEL) calculated of \(\delta^{2}\)H = 6.9 \(\delta^{18}\)O + 3.4 (Figure 7). This relationship closely compares to the LEL found for Lake Chilwa and basin stream water by Missi and Atekwana [18]. Lake Chilwa data for the 2019 wet season are also shown by way of comparison (Figure 7) and correspond to samples from the rainy season that immediately followed lake desiccation. The expectation is that lake water sampled was from direct precipitation and may account for the highly depleted signatures observed. In contrast, river (plotted'surface water') isotopic signatures comprise current precipitation and groundwater (baseflow/interflow) contributions, the latter primarily sustaining dry-season river flows.  
Overall, the range in river signatures is consistent with local precipitation variation recently evaluated throughout Malawi (our on-going work) and supports that local precipitation contributes largely to river flows, particularly during the wet season. Groundwater isotopic signatures, similar  
Figure 7: Plot of \(\delta^{2}\)H versus \(\delta^{18}\)O for groundwater, surface water and Lake Chilwa isotopic samples.  
to surface water, cluster around the LMWL but exhibit a more restricted range (\(\delta^{2}\)H range from \(-\)38.8 to \(-\)28.6%o, with a mean of \(-\)32.7%o; and \(\delta^{18}\)O range from \(-\)6.2 to \(-\)4.7%o, with a mean of \(-\)5.6%o). Deuterium excess ranges from \(-\)8.48 to 12.3%o, with a mean of 10.3%o. This provides strong evidence of non-evaporated precipitative areal recharge into the groundwater system. Overall, the groundwater and surface water datasets show a consistent isotopic relationship (plotting on a common area) both during the dry and wet seasons. During the wet season, the scenario is indicative of influent seepage mainly of mid to lowland rivers, while during the dry season, it is suggestive of groundwater contribution to river flows. There is no clear indication of flooded lake recharge of groundwater at lake edges evident of inconsistent isotopic signatures, ascribed to dominant isotopic signature contribution by precipitation and evaporative fractionation (recognising though monitoring points are unlikely to be close enough to the lake to show that local interaction).