An evaluation of three evapotranspiration models to determine water fluxes over hillslopes encroached by invasive alien plants in Eastern Cape Province, South Africa

Ecosystem services in the South African grassland biome are being impacted by the presence of invasive alien plants (IAPs), particularly from the Australian genus Acacia. IAPs have elevated landscape water use and determining water fluxes is crucial to defining suitable interventions. This study evaluates three models of water flux over IAP-invaded grassland against evapotranspiration (ET) measured by a large-aperture scintillometer (LAS). ET was modelled using an energy balance model (MEDRUSH), a biophysical model (Penman-Monteith-Leuning (PML)) and a remotely sensed product (MOD16), and their results compared with ET measured by the LAS. ET_LAS was measured during an abbreviated field campaign in November 2019 over a dense silver wattle (A. dealbata) stand associated with hillslope seeps. Of the three models tested against the LAS campaign, MEDRUSH performed well-enough to be used to model ET over the continuous scientific-grade micro-meteorological record (315 days). We used MEDRUSH to model ET over the invaded hillslope seeps and compared this with the expected ET that would occur over the adjacent, un-invaded grassland. Total ET over the IAP-invaded area was approximately 536 mm (60% of reference ET) as compared with the grassland (202 mm), showing that woody encroachment significantly increases landscape water use. To estimate the local impact of this, we used earth observation to determine the area of woody invasion in a 2 664 km² regional window. Approximately 10% (274 km²) of this window was invaded by IAPs and, assuming the geographic window was representative of all the areas invaded by wattle, this represents ~ 89 000 ML of water used annually by the IAPs. Removal of wattle, followed by suitable restoration of landscape functionality, would greatly enhance rangeland productivity and water production from hillslope seeps.