Assessing the contribution of hydrologic and climatic factors on vegetation condition changes in semi-arid wetlands: An analysis for the Narran Lakes

Abstract

Vegetation in semi-arid wetlands often serve as a critical habitat and refuge for a wide range of species. In many wetlands, on-ground monitoring of vegetation is either not comprehensive or unavailable, which impedes our understanding of the system. However, basic data on climate and hydrological variables, as well as remote sensing data, can often be acquired. The Narran Lakes system in the Lower Balonne catchment in New South Wales, Australia, is a Ramsar-listed wetland and an exemplar in terms of possessing such basic data. For the Narran Lakes we conducted correlation analysis between the anomaly of the Normalized Difference Vegetation Index (NDVI) as response variable and various climatic and hydrological factors as explanatory variables taking into account different time-lag and accumulative time-averaged effects. The generalized additive model framework was used to identify the contribution of the individual variables to NDVI and examine the nonlinear interactions of the hydro-climatic, water availability factors (soil moisture, precipitation and inflow in the study) on NDVI change within the Narran Lakes. We also undertook various cross-validation exercises to appreciate uncertainties in the results. The results show that: (1) Soil moisture is the primary factor influencing NDVI; and (2) Water availability factors interact in a complex manner to affect NDVI and, more specifically, these factors have a positive impact on NDVI, although the degree of impact differs; (3) The impact of the hydrological and climatic factors is highly variable between wet and dry resource states, both for the whole floodplain vegetation and its lignum community. Overall, the analysis improved our understanding of how the driving factors affect vegetation growth, thus supporting the monitoring and management of vegetation communities in the Narran Lakes. The methods can be applied to other wetlands with similar data availability.