Measuring Water Requirements Of Riparian Regions in the Southwestern U.S. Compared with Drylands in Australia

Science Center Objects

Floodplain red gum forests are sites of high biodiversity in arid regions of south Australia. They depend on periodic floods from rivers, but dams and diversions have reduced flood frequencies, leading to deterioration of the trees. We determined the water requirements of red gum trees so environmental flows can be used to restore and maintain the forests. We used measurements of transpiration of red gum and an algorithm based on Moderate Resolution Imaging Spectrometer (MODIS) vegetation index to estimate total evapotranspiration (ET) for a large area of forest fed by the Murrumbidgee River. MODIS red gum ET estimates adequately predicted sap flux estimates. From 2000-2012, MODIS ET estimates were the same as water balance estimates determined from river gauges. However, during the 2000-2009 drought, MODIS overestimated ET compared to water balance data due to water stress detected in sap flow studies. We extended ET back to 1995 using Landsat imagery and gauge data. Over the 38,134 ha of continuous forest, ET consumed 11% of river water during the wet years of 1990-1997 while during the drought years of 2000-2009, 35% of inflows were consumed.

Background & Importance

Tropical bird in red gum tree in southern Australia
Tropical bird in red gum tree in southern Australia (2013). (Credit: Pamela Nagler, USGS. Public domain.)

Floodplains and their riparian forests and wetlands are globally important reservoirs of biodiversity. They are especially important in dryland ecosystems where rainfall is scarce and rivers originating in wetter areas are the main source of water supporting mesic vegetation and associated fauna. However, dryland rivers are susceptible to degradation due to diversion of flows for human use and regulation of the natural flood regime to protect property. In response, land managers in several countries have turned to environmental flows to attempt to recreate conditions conducive to ecological restoration of regulated rivers. It has been difficult to evaluate the effectiveness of environmental flows, which typically are much lower in volume than historic natural flows. Developing strategies for environmental flows requires knowledge of how plant communities respond to the timing, volume, duration and frequency of natural floods, but this knowledge is often lacking.

A case in point is the Murray-Darling River Basin in southeastern Australia. The riparian zone is dominated by riparian forests consisting of flood-and-drought tolerant Eucalyptus and Acacia trees, plus understory grasses and seasonal emergent wetland areas. These woodlands support a highly diverse and unique community of reptiles, birds and mammals. Before human intervention, the Murray-Darling River system had highly variable flows, with cycles of floods and drought driven by variations in rainfall in the Great Dividing Range of mountains to the east. The biota of the basin was adapted to these cycles, and key ecosystem processes depend on both floods and dry periods for maintenance.

After 100 years of river regulation and diversion of water for agriculture, flood events are of lower volume, duration and frequency than in previous years. Degradation of the riparian forests in the basin has been noted for some time, but concern was especially high during the Millennium Drought between 1997 and 2010. In response, the Murray-Darling Basin Plan was implemented, which included environmental flows into the riparian zone.. A specific goal was to provide a flood regime such that 80% or more of the iconic red gum (Eucalyptus camaldulensis) forests and woodlands would be restored to good condition (MDBA, 2010). Efforts are underway to try to define the set of flood regimes that meet this goal.

Murray River and its riparian vegetation in southern Australia
Murray River and its riparian vegetation in southern Australia (2013). (Credit: Pamela Nagler, USGS. Public domain.)