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. , "" -"') .~ Biological issues, comment to SWSI, August 2004 21 those served by irrigation ditches~ He also describes the probtems of applying existing GIS. based analyses to conservation planning, such as the federal GAP analysis4 His forecast of development along the river valleys is chilling (see Theobald 2003 for graphics)~ New management such as intermittent leasing involving ditches could bring some unpleasant surprises. Because there is very little irrigation directly from mainstems, water is diverted, applied, partly returnsf and is diverted again, the amount that flows through structures may be very large; it is 2.1 million acre-feet in the South Platte, along with 880,000 AF of groundwater, that irrigated about 1.1 million acres in 1998, before the drought and the competition for water got serious (Colorado Water Conservation Board River Basin 'act sheets..~ on website). It is unlikely that most of that will be taken, but seemingly small removals might be locally very important. For comparison, recall that 2.1 minion acre-feet of diversion is about five times the amount of flow at the state line. The mileage of dttches and canals wUI exceed that of the mafnstems on the Plains by a considerable extent~ This is not a trivial part of the landscape. Because of the way ditches work. physically and sociaUYi it is probable.that the change's would be concentrated in a few areas, rather than evenly spread.. Many ditches literally cannot modify flows for individuals; they cannot make changes at scales smaller than a lateral (the last subdivision of flow before directing it into farms) (Wiener 2003, 2004). In a market system, the water most likely to be leased away ought to be from lands and irrigation systems that are least profitable, which may be those with larger inefficiencies of convey~nce and application - which are likely to be those with the longer distances and longer areas affected by seepage and leaks, and those wtth the larger areas unintentionally irrigated by return flows, - though this generalization is not proven or easily provable yet. If this is so, it may mean that _ disproportionately large environmental change would result from rational water management, as economic inefficiencies affect the workings of the market. The net may be that intent to transfer 5% of agricultural water for urban uses could mean cutting off more than 100/0 of diversions to more than 100/0 of ditch mileage, and 'more than 10% of return-flow-irrigated lands (often the biologically valuable area between the farming and the River r in bottomland and floodplains). The neXt ,complication is that we may have litt'e ability to predict consequences of such changes. Mainstem rivers on the Plains and in some of the intermountain Western basins are alluvial, so that the surface flows are roughly the top of the slower and larger subsurface flows4 Drying the surface flows may not terminate sub-irrigation of riparian areas; tap-rooting trees and other vegetation may be littJe-affected. But ditches and canals are not entirely like that; some have been cut through impeNious strata, to follow the necessary gradients for water and sand transport. That means that stopping diversion can lead to thoroughly-drying up some ditch- supported riparian areas rather quickly, and frequent changes could make permanent changes in vegetation where invasives and other species can out-compete those which presently provide desirable environmental services. Interruptible irrigation might aJso interact with control of invasives such as tamarisk, which is particularly problematic for Colorado (see T amariskcoaUtion.org and <http://www..coopext.cotostate.edulTRAJtamarisksymposium.html> ). CUmate-responsive management coulct also be part of the picture, increasing the transfers in times when environmental stress is alr~ady high. A long quote Bin summary, hydrology has been modified in all wetland types throughout the Great Plains, which has had a significant effect on vegetation composition on a continental scale~ Historically, although the distribution and number of wetland types varied among regions. wetlands exhibiting different hydroperiods were located ,in close juxtaposition on a regional or local scale. Ephemeralt temporaryt and seasonal wetlands supported rich communities of annuals (and varying amounts of some biennials) with some perennials that provided a rich food base in the form of seeds and substrates for invertebrates. By contrast, semipermanent and permanent basins were largety dominated by perennials, except in drought years when mudflats