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United States Department of Agriculture

Agricultural Research Service

Title: Spatial/temporal Analysis of the 1999-2004 South Gila Depth to Water Table and 1995-2002 Groundwater Discharge Data

Authors
item Lesch, Scott - U.C. RIVERSIDE, CA.
item Corwin, Dennis
item Suarez, Donald
item Suarez, Donald
item Chakravartty, A - U.C. RIVERSIDE, CA

Submitted to: Laboratory Publication
Publication Type: Other
Publication Acceptance Date: November 5, 2005
Publication Date: December 7, 2005
Citation: Lesch, S.M., Corwin, D.L., Suarez, D.L., Chakravartty, A. 2005. Spatial/temporal analysis of the 1999-2004 south gila depth to water table and 1995-2002 groundwater discharge data. Laboratory Publication. Report for Cooperative Project. pp: 1-127.

Interpretive Summary: This report summarizes the spatial-temporal, statistical analysis of the 1999-2004 South Gila monthly depth to water table (groundwater) data and 1995-2002 discharge station pumping data. This analysis has been performed as part of the ARS research agreement # 5310-13610-013-13R, an inter-agency agreement between the U.S. Bureau of Reclamation (Yuma Area Office) and the USDA-ARS George E. Brown Jr. Salinity Laboratory to provide a spatial-temporal, statistical analysis of the South Gila groundwater data. The statistical analyses presented here indicate that the average groundwater level has exhibited a stable, seasonal cycle over the last five years. The specific average monthly estimates exhibit twice yearly cycles that closely correspond to the seasonal South Gila lettuce and winter wheat cropping patterns. Additionally, the majority of groundwater variation can be classified as spatial in nature and the current network of wells already appears to be sufficient for defining the general spatial pattern across the district. Unfortunately, the current network also appears to be highly inadequate for predicting short term, spatially referenced deviations in the depth to water table pattern. Furthermore, detailed analysis of the groundwater data from each well suggests that a small number of wells may be malfunctioning and/or excessively responding to nearby surface water sources. The results from a formal volume/concentration analysis suggest the discharge water quality is stable over time. Additionally, there appears to be no statistically significant relationship between the volume of discharged groundwater and the measured EC levels at each station during this eight year timeframe. Approximately 98% of the variation in water quality can be attributed to between-station effects. Finally, the results from a dynamic, time series regression model indicate that it takes about 4 to 8 weeks for the pumping stations to directly impact the depth to water table level. This model suggests that an additional 250 acre-feet of groundwater must be discharged for two consecutive months in order to lower the water table by about one inch. This model also suggests that a pro-active pumping strategy might be employed to mitigate the seasonal effects while simultaneously holding the annual cumulative discharge amount to a desired target level. In other words, by anticipating and instigating optimized monthly target discharge volumes, the Bureau should be able to significantly reduce (i.e., dampen down) the cyclic seasonal pattern currently observed across the South Gila district. This latter point is particularly relevant, since the majority of wells in the South Gila district that exhibit shallow water table levels also tend to be the wells that exhibit the most pronounced seasonal effects. Such a pro-active pumping strategy would not require any additional discharge (on an annual basic) above the current level. Presumably, such a strategy would also not result in significant additional operational costs, since the average annual pump operation times would not change.

Technical Abstract: This study examined the spatial-temporal patterns in the 1999-2004 South Gila monthly depth to water table (groundwater) data and 1995-2002 discharge station pumping data. This analysis was performed as part of the ARS research agreement # 5310-13610-013-13R, an inter-agency agreement between the U.S. Bureau of Reclamation (Yuma Area Office) and the USDA-ARS George E. Brown Jr. Salinity Laboratory to provide a spatial-temporal, statistical analysis of the South Gila groundwater data. Our analyses indicate that the average groundwater level has exhibited a stable, seasonal cycle over the last five years. The specific average monthly estimates exhibit twice yearly cycles that closely correspond to the seasonal South Gila lettuce and winter wheat cropping patterns. Additionally, the majority of groundwater variation can be classified as spatial in nature and the current network of wells appears to be sufficient for defining the general spatial pattern across the district. The results from a formal volume/concentration analysis suggest the discharge water quality is stable over time. Currently, there appears to be no statistically significant relationship between the volume of discharged groundwater and the measured EC levels at each station during this eight year timeframe. Approximately 98% of the variation in water quality can be attributed to between-station effects. Furthermore, the results from a dynamic, time series regression model indicate that it takes about 4 to 8 weeks for the pumping stations to directly impact the depth to water table level. This model suggests that an additional 250 acre-feet of groundwater must be discharged for two consecutive months in order to lower the water table by approximately one inch. This model also suggests that a pro-active pumping strategy can be employed to mitigate the seasonal effects while simultaneously holding the annual cumulative discharge amount to a desired target level. Such a pro-active pumping strategy would not require any additional discharge (on an annual basic) above the current level. Presumably, such a strategy would also not result in significant additional operational costs (since the average annual pump operation times would not change) while facilitating a potential decrease in crop yield losses due to shallow water table conditions.

Last Modified: 12/26/2014
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