Complicated ecological tradeoffs from water & agriculture

A recent paper in Water Resources Research (2006: 42) by Eloise Kendy and John Bredehoeft Transient effects of groundwater pumping and surface-water-irrigation returns on streamflow shows how a long history of excess irrigation in the US west has prodcued streamflow that ecosystems and people have come to rely upon. Now improvements in irrigation effectiveness (i.e. more crop per drop) could reduce this streamflow. These connections show how complicated tradeoffs between different water uses can become. Fortunately, in this case, as in many others, it appears that more sophisticated water management can reduce the intensity of this tradeoff.

Abstract: In surface-water-irrigated western valleys, groundwater discharge from excess irrigation sustains winter streamflow at levels that exceed natural flows. This unnatural condition has persisted for so long that hydrologists, water managers, and water users consider it to be normal. Changing land uses and irrigation practices complicate efforts to manage groundwater discharge and, in turn, to protect instream flows. We examined the impacts on streamflow of (1) seasonal groundwater pumping at various distances from the Gallatin River and (2) improving irrigation efficiency in the Gallatin Valley, Montana. We show that the greater the distance from a seasonally pumping well to a stream, the less the stream depletion fluctuates seasonally and the greater the proportion of annual depletion occurs during the nonirrigation season. Furthermore, we show that increasing irrigation efficiency has implications beyond simply reducing diversions. Improving irrigation efficiency reduces fall and winter flows to a lower, but more natural condition than the artificially high conditions to which we have become accustomed. However, existing water users and aquatic ecosystems may rely upon return flows from inefficient irrigation systems. By strategically timing and locating artificial recharge within a basin, groundwater and surface water may be managed conjunctively to help maintain desirable streamflow conditions as land uses and irrigation practices change.

Eloise Kendy has a short related article in Geotimes (June 2005) Water woes: predictable but not inevitable, where she writes how land-use change produces inadvertent ecological engineering that should become more intentional and less haphazard.

The change from irrigated agriculture to residential development entails more than simply pumping groundwater. Most irrigation systems in the West — especially the oldest systems on the most productive ground — use diverted surface water. Irrigation water that crops do not use seeps into the soil and eventually reaches the water table, where it recharges groundwater in the underlying aquifer. So-called irrigation return flow is a major source of groundwater recharge in irrigated western valleys.

The irrigation-charged groundwater slowly makes its way underground to rivers, streams and springs, where it eventually discharges. Groundwater discharge from irrigation return flow keeps rivers flowing well into late summer and fall, even after all the snow has long since melted, even after the rains have stopped. Although not a natural phenomenon, we consider this annual flow pattern “normal,” for it has recurred for more than 30 years.

The most important hydrologic change brought on by urban and suburban development is a drastic reduction in groundwater recharge. Urban land surfaces such as roofs, roads and parking lots are impermeable. Rain and snowmelt run off these surfaces, instead of seeping into the ground and recharging aquifers. In a typical engineering design, runoff is quickly shunted into the nearest stream or river to rid the area of potential flood waters. Consequently, localized recharge greatly decreases, streamflow becomes “flashier” (larger fluctuations over shorter periods of time), and late-season, groundwater-fed streamflow decreases. When irrigation stops, seepage from excess irrigation water also stops, or continues to recharge the aquifer only from leaky ditches.

Almost without exception, rural residential development in the West relies on well water for domestic use. So, on top of reducing aquifer recharge, the change from surface-water-irrigated cropland to groundwater-irrigated yards increases aquifer discharge. Less water goes into the aquifer than before, and more water goes out.

Previously, irrigation diversions depleted streamflow in the spring and early summer, and irrigation return flow maintained streamflow well into the late summer and fall. Now, with fewer surface-water diversions, early flows increase, as does the risk of flooding. Conversely, late-season flows decrease, potentially leaving fish and downstream irrigators high and dry.

When sewers were put in place in Long Island, N.Y., in the 1950s, wastewater that previously recharged the aquifer now discharges straight into the ocean. The loss of aquifer recharge caused the water table to drop about 20 feet. To save the aquifer, more than 3,000 small recharge basins were constructed. Their average combined infiltration rate of 150 millions gallons per day has successfully reversed the trend of declining water levels in the aquifer.

Out West, many creative options exist for water management. Most of the basins within the Basin and Range province, which, loosely defined, extends from Canada to Mexico, provide ideal geologic settings for storing artificially recharged water underground. Using existing irrigation infrastructure, we could spread spring runoff onto benchlands, allowing it to flow underground toward rivers, where it would replace irrigation return flow as a resource for late-season use. Another simple option is to discourage landscaping that requires irrigation.

via Kevin Vranes’s No Se Nada.

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