FLOOD01774 - Laserfiche WebLink

Home Browse Search

surface-water applications for irrigation are made through diversions out of the Fort Lyon Canal into local laterals that deliver water to fields. Usually, the water is used to flood the fields. In this respect, the Fort Lyon Canal represents a source of recharge to the local aquifer. Goff and others (1998) computed annual surface-water applications using an algorithm that was based on the amount of water in the canal and the ratio of irrigation-program participants in the study area to the total number of participants who are supplied by the canal. The results of their estimates are listed in table 4. Surface-water applications were greater than the long-term mean during most of the 1980's, similar to flow in the Fort Lyon Canal, which generally was high during that decade. After the 1980's, surface- water applications decreased and returned to levels slightly higher than those preceding the 1980's. Ground-Water Withdrawals When necessary, generally when water is not available from the Fort Lyon Canal, ground water is routinely withdrawn from the local aquifer for irriga- tion purposes to supplement irrigation made with surface-water diversions. Ground water also is with- drawn for public water supply. These withdrawals represent discharge from the aquifer. Irrigation wells are distributed throughout the study area in a relatively uniform fashion; however water-supply wells, oper- ated by the city of La Junta are clustered in an area of La Junta on the north side of the Arkansas River. Ground water is withdrawn for irrigation throughout the study area and is usually used to flood fields, although some alternative methods, such as spray and drip irrigation, are used. The records available for analysis in this study are from a previous study (Goff and others, 1998) and are summarized in table 4. They are annual records and are based on reported with- drawals from the city of La Junta and on a power conversion method (Boyle Engineering Corp., 1990) for irrigation wells that estimates the amount of water withdrawn at individual wells based on electric power meter readings. The Colorado Division of Water Resources has recently begun to maintain records of withdrawals made by irrigation wells. The records are for indi- vidual wells and have more detail than is available from the historical records. These contemporary records should be very useful, in the future, for charac- terizing the amount of ground water withdrawn for irrigation. The records from Goff and others (1998) indi- cate that ground-water withdrawals generally have decreased in the study area (table 4). In 1972 and from 1974 through 1978, withdrawals were greater than the long-term mean and increased steadily to a high for the period of record. After a peak in mid 1976, decreases began and continued through about 1987, after which ground-water withdrawals became rela- tively steady at a level less than the long-term mean. The records indicate that ground-water withdrawals at the end of the period of record were much less than the withdrawals made in the late 1970's and early 1980's. The records also indicate that although ground-water withdrawals in the study area sometimes have a nearly one-to-one inverse relation to surface-water appIica- tions, such as in the late 1970's and early 1980's, the relation is not necessarily constant, such as in the period after the mid- 1980's when surface-water appli- cations are nearly constant and ground-water with- drawals tend to increase. Channel Processes As stated previously, the Arkansas River and the surrounding alluvial aquifer can exchange water. When the river is recharging the aquifer, the amount of recharge increases as the level of the river increases. In the short term, increases in river level, or stage, are related to changes in rates of flow. For instance, flow typically increases when there are widespread rains. In the long term, however, the level of the river may change due to increases in the elevation of the river bed. Long-term changes in the river-bed elevation affect ground-water discharge, or drainage, to the river. In general, ground water at elevations lower than the river bed will not drain from the system and the river bed forms a base level for ground water. Walls and Lunsford (1992) documented a general increase in the elevation of the river bed for the Arkansas River at La Junta by evaluating the eleva- tion of the point of zero flow, which is determined routinely as a part of maintaining streamflow records. In this study, a similar method was used to evaluate changes in river-bed elevation. The method used in this study, sometimes referred to as the "specific-stage method" HYDROLOGIC FACTORS THAT AFFECT GROUND.WATER LEVEL.S 21