Arid Land Rivers – Boom and Bust – or Buggered?
Dryland rivers flow from more than half of the world’s lands and support nearly 40 percent of the world’s population, yet these ecosystems are very much understudied.
While some dryland rivers are strongly dependent on snowpack or glacial melt in mountainous headwaters (and therefore have relatively stable base-flow conditions), many sit at the extreme end of the spectrum when it comes to variability in river flow. These rivers can go from years of no flow to massive floods that persist for days to months. Many of these highly variable rivers are found in Australia, but the deserts of southern California and the southwestern United States also show similar characteristics. Dryland rivers are often ecosystems with characteristic boom and bust ecologies. The plants and animals in these dryland rivers have evolved ways to survive the bad times and take full advantage of the good times. The most variable rivers in terms of flow are in outback Australia and other extreme deserts of the world.
Recently, Interim Delta Lead Scientist Cliff Dahm gave a plenary presentation on dryland rivers at the joint meeting of the American Society for Limnology and Oceanography (ASLO) and the North American Benthological Society (NABS) in Santa Fe, New Mexico-one of the most water-efficient cities in the United States. Dahm’s joint plenary talk was presented with Stuart Bunn, Director, Australian Rivers Institute, Griffith University, Queensland, Australia, who is a leading researcher on dryland rivers in Australia.
“Many dryland rivers are associated with vast floodplain wetland ecosystems that support large populations of fish, migratory birds and other wildlife during seasonal or episodic flood events,” Dahm and Bunn said. For much of the time, however, dryland rivers consist of a chain of fragmented waterholes or isolated springs. These provide important refuge for aquatic organisms (plants and animals) during protracted intervals between flows, as well as sustain terrestrial (land) wildlife and human settlements.
When flood waters come, the floodplains of dryland rivers enjoy impressive bursts of productivity by the fascinating and well adapted plants and animals that are found there. Persistent waterholes during the dry periods and the location of permanent springs are critical refuges for many aquatic plants and animals. Mapping locations of permanent waterholes and perennial springs is a critical step in protecting these sensitive environments. Recently, the location of many of these permanent springs has been shown to be linked to tectonic processes in both the western United States and the mound springs region of central Australia. Waters from deep in the Earth provide a stable supply, albeit sometimes of variable quality, for these permanent springs.
Dryland rivers in arid and semi-arid regions of the world generally lose water along their flowpaths. The amount of water lost from evaporation, plant transpiration, groundwater recharge, and human use is important to understand when deciding upon water allocations in these commonly water-limited environments. Recent research has focused on better quantification of the water budgets for dryland rivers throughout the world. One attribute of dryland rivers that is increasingly understood is that these ecosystems are very efficient at retaining and using nutrients. When dryland rivers are compared to other rivers in the world, the dryland rivers retain nitrogen even in highly populated and agriculturally rich regions much more efficiently than their counterparts in wetter regions of the world.
Dryland rivers, however, are also some of the most “buggered” rivers in the world. Buggered is slang in Australia for messed up or impaired, although the word also sometimes has a more colorful meaning. “Water diversion remains the single biggest threat to dryland rivers and overexploitation of water resources has led to spectacular wetland losses in some systems,” Bunn said. “Even modest levels of water abstraction, especially during dry periods, can lead to increased fragmentation of aquatic habitat and loss of permanent refuges.”
Current pressures on dryland rivers, waterholes, and springs include floodplain water harvesting (common in Australia as this water is not regulated), floodplain agriculture, direct pumping from springs and waterholes, and urban water use. Floodplain water harvesting means that whenever flood flows in these rivers leave their channels and move out onto the floodplain they can be captured and stored in earthen dams for later use by agriculture. Future pressures include a warmer climate with more variable precipitation. Projections for the Murray Darling Basin in southeast Australia are for less surface water available and for longer dry spells. Similar predictions for much of the southwestern United States would move these dryland rivers towards the more variable flow conditions currently found in the dryland rivers of Australia.
There are many science challenges faced in studying dryland rivers. One is the mismatch between modeling capability for understanding human needs for water (good) and the needs of the environment (poor). Another is the current gauging networks that are designed to measure flows at higher discharge but do a poor job of measuring low flows and intermittent flows. A possible solution to better flow gauging in dryland rivers is to bring together conventional measurement of surface hydrology with remote sensing, depth sensors, and geochemistry. This technology would help define the distribution and persistence of aquatic ecosystems on the landscape through time.
There are also societal challenges to protecting and preserving dryland rivers. One example is on-farm efficiency in the use of water. The decade from the early nineties to the beginning of the 21st century saw a 50 percent decrease in water used per hectare (2.47 acres) of farmland in Australia to sustain comparable crop production. The United States saw a 10 percent decrease during the same period of time. More efficient use of water for agriculture is critical to protect and preserve dryland rivers.
More efficient urban water use is another societal challenge. Urban water use in southeast Queensland from 2000-2008 during a severe drought dropped from 87 gallons per person per day to 30 gallons per person per day. After the end of the drought, urban water use remains at about 40 gallons per person per day. These water use values for Australia can be compared to relatively water-use-efficient cities in the southwestern United States like Santa Fe, New Mexico and Tucson, Arizona where per person water use is about 100 gallons per person per day. San Francisco is currently the most efficient larger city in California with use of about 105 gallons per person per day. Some cities in southern California still use up to 400 gallons per person per day. The average in Sacramento is currently about 260 gallons per person per day.
Dryland rivers are important aquatic ecosystems worldwide. They sustain fascinating plants and animals that are adapted to highly variable availability of water. Protecting and preserving these rivers will require societal commitments to efficient water use in agriculture and urban areas that allow adequate water to keep dryland rivers viable.