Laserfiche WebLink
• Bridgeport to Lisco <br />. • Lisco to Lewellen (Inflow to Lake McConaughy) <br />• Keystone (Outflow from Lake McConaughy) to Sutherland <br />• Sutherland to Maxwell <br />• Maxwell to Cozad <br />• Cozad to Overton <br />• Overton to Kearney <br />• Kearney to Grand Island (Two day travel time) <br />Before the summer of 2001, PWAP calculations ended at Odessa. There is now the need to track <br />water to Grand Island. As more information on travel times below Overton become available it <br />may become necessary to install a new gage in the reach. <br />Conveyance Losses <br />The conveyance losses were calculated by multiplying the average number of open water acres <br />in the reach by the monthly average open water evaporation rate. The island CL value is <br />calculated by multiplying the average island acres multiplied by the average monthly island <br />evapotranspiration rate. The open water and sandbar area is considered an average because <br />depending on the flow, there will be either more open water and less sandbar area or less open <br />water and more sandbar area. The average monthly open water evaporation rate was calculated <br />using 20 years of pan evaporation data, 1980 to 1999, from the more reliable weather stations <br />. along the river, North Platte and Grand Island. The open water evaporation is equal to the pan <br />evaporation data multiplied by a standard factor of 0.7. <br />The average island evapotranspiration rate is much more difficult to determine. An American <br />Society of Civil Engineers report called Water Use by Naturally Occuring Vegetation Including <br />an Annotated Bibliography sums up the problems with the following statement: <br />In summary, the process of estimating or determining evapotranspiration of naturally occurring <br />vegetation is fraught with uncertainty. Not only is it difficult to accurately determine the <br />appropriate water use, but it is even more difficult to extrapolate the results to larger areas. <br />Winter and growing season factors were estimated using data found in that report and other <br />sources. The factors are estimated ratios of the island evapotranspiration to open water <br />evaporation and are dependent upon the soil type, depth to water, and the type of vegetation <br />present. A table of vegetation common in the area and the associated average annual <br />evapotranspiration can be found in Appendix A. The winter factor used is 0.5 and the growing <br />season factor is 0.8. A sensitivity analysis showed that changes to these numbers do not <br />significantly affect the resultant monthly evapotranspiration loss values and that the greatest <br />expected change was less than 5 cfs per tenth of change. <br />State Line — Lewellen Calculations <br />The North Platte Decree sets the CL values for these reaches for the months of May to <br />September. The negotiated values were based on river surface area, average evaporation, and an <br />• evapotranspiration factor. In order to have October to April values consistent with the decreed <br />2 <br />