Role of landscape changes in water consumption for water policy in humid environments

While most water disputes among States and other stakeholders have occurred in the arid West, increasing population growth and irrigation expansion in the Southeast have sparked contentious disputes among States. For example Alabama, Georgia and Florida (AFG) have been involved in negotiations to develop water allocation formulas for rivers crossing the three states since 1998. This dispute has involved millions of dollars by state and federal agencies, industry and municipalities potentially impacted in the suits and negotiations have involved the highest level of government attention at both the State and Federal level. The initial dispute and need for a water compact was driven by concerns by Alabama and Florida that withdrawal of water by metropolitan Atlanta would harm downstream municipal and industrial water availability, fresh water and estuarine habitats, navigation and hydroelectric generation. As discussed below, in humid environments the uptake and evaporation of water by natural deep rooted vegetation is a major part of consumptive water use. As the natural landscape is transformed to an urban landscape there is substantial evidence that local water consumption is decreased and run-off increased Thus, though ironic, it is probable that increasing the size of urban areas in humid environments actually increases stream flows on an annual basis. Whether, the reduced consumption in urban areas actually increases stream flows during extreme drought periods is uncertain at the present.

In addition to urban landscape change, seasonal agricultural crops may also consume less water in humid environments even with irrigation than the natural vegetation. We note that though crops may consume less water than natural vegetation or farmed timber there are other reasons for maintaining these ecosystems. But, the intuitive belief that irrigation would consume additional water may not be true in humid environments. While some of these aspects of water consumption due to anthropogenic landscape change are well known to hydrologists and atmospheric scientists, variations in water consumption due to landscape change evidently have not been considered in the policy dispute between Alabama, Florida and Georgia.

Despite the large annual average precipitation in the Southeast and relatively uniform annual distribution of precipitation, the uptake and evaporation by natural vegetation in the region leads to very low flow rates in streams and rivers in the summer. During drought years these summer and early fall stream flows can reach critically low levels endangering fresh water habitats and municipal and industrial withdrawals. Thus, not withstanding the relatively small annual fractional consumption by Atlanta, Alabama and Florida have targeted Atlanta's withdrawal as a major culprit in maintaining minimum flows in rivers. While the primary focus has been on Atlanta's withdrawal, a significant irrigated agricultural system has developed in the last thirty years in South Georgia. This system is primarily based on an on-demand (summer) withdrawal from ground water and surface water.

During the negotiations and suits related to the AFG the emphasis has been on withdrawals not landscape changes. For landscape change to be a part of the policy debate on water policy in humid environments additional quantitative information is needed. Quantitative details on how local consumption is reduced as natural vegetation is transformed into an urban landscape and whether this reduced consumption actually increases run-off (in drought conditions) is needed. There are also policy issues relative to base line dates, reservoir control etc. on how land use changes might be counted as part of the water budget under the compact negotiations or court settlements.

The present paper will address initial hydrological data and information related to water consumption in the Southeast. Second, it will outline the type tools needed to make the quantitative calculations mentioned above. In the past the AFG dispute has been based only on precipitation input and metered withdrawals in part because quantitative information on evapo-transpiration was lacking.

As mentioned above, it is almost certain that transformation to an urban landscape has decreased consumption of water as deep-rooted transpiring vegetation (primarily deciduous forests) has been replaced with urban surfaces and grasses. It is also clear based on hydrological records that non-porous urban surfaces increase both short-term and annual average run-off during precipitation events. However, in terms of water consumption it is not as clear on whether the decreased urban consumption during drought conditions actually increases stream flow during these critical low flow events. For example the decreased consumption by the urban area may increase soil moisture levels but this may be consumed by the remaining urban vegetation or surrounding vegetation before it actually reaches a stream.

Developing the tools needed to provide information on water consumption in humid environments is critical to making informed water policy decisions in the Eastern U.S.. Additionally, such policy has national implications on whether the Southeast might be able to balance agricultural production in the arid West as urban demand and water supply issues contract irrigated production in the West.