Díaz-González, Manuel F.

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    Estimation of water balance and groundwater processes of the Salinas to Patillas area in southeastern Puerto Rico for 1980 - 2010
    (2012) Díaz-González, Manuel F.; Pérez-Alegría, Luis R.; College of Engineering; Segarra García, Rafael; Zapata López, Raúl E.; Department of Civil Engineering; Arroyo Caraballo, Jose R.
    This study describes the groundwater recharge and pumpage, and surface water distribution through a water balance model for the Salinas to Patillas agricultural area located over the alluvium of the eastern part of the Puerto Rico South Coast Aquifer. This area is influenced by the Patillas-Guamaní irrigation canals of the Guayama Irrigation District which is a subdivision of the Puerto Rico Electric and Power Authority (PREPA). The combined soil water and groundwater budgets are based on data collected during the last 31 years, from 1980 to 2010. It is believed that recharge is as dependant on annual precipitation distribution as it is to annual total precipitation. It was estimated that the average annual percolation from precipitation from 1980 through 1993 was 15 percent of the total rainfall while 1994 through 2010 was 11 percent. Percolation from irrigation gradually decreased from the early 1980s to the mid 2000s as irrigation efficiency gradually increased during this period and irrigated area decreased. Based on water deliveries data by PREPA and irrigation efficiency estimates, it is theorized that in furrow irrigation fields about 20 percent of the irrigation water applied actually recharge the aquifer, where other 10 to 40 percent of the water that may recharge the aquifer in irrigated fields is the direct result of the interaction between precipitation and irrigation. In the same way, it is theorized that none of the water imported for drip irrigation percolates and that in sprinkler irrigated fields only about 5% of the imported water percolates, where all other percolation come from the interaction with precipitation. For Guayama, Arroyo and Patillas, the general order of importance for groundwater recharge inputs was estimated as precipitation, streams, canals, irrigation and dams. In the Salinas area was the stream percolation the most important input, followed by precipitation, canals and irrigation. Irrigation canals are a very important source of groundwater recharge because of its continuous and safe input to groundwater. Estimated groundwater recharge from the two reservoirs within the study area was determined as negligible. The Salinas to Patillas area experienced a reduction in groundwater pumpage after the sugarcane production stopped in the area due to the reduction in irrigation water requirement and in irrigated area. The greatest reduction in groundwater pumpage prior and after the end of sugarcane production was experienced by Arroyo (49 %) while Guayama experienced the least changes (11 %). Reduction in groundwater pumpage in Patillas and Salinas was estimated to be 27 and 14 percent, respectively. In general, results from the model show a healthy aquifer in the Arroyo, Patillas and Guayama area in terms that net groundwater recharge where the vast majority of the years counted with average net recharge rates greater than 180 mm/yr. Results indicates that in the Salinas area the average net groundwater recharge was around 64 mm/yr and groundwater depletion to the point of possible saltwater intrusion occurred during the years 1980, 1983, 1991, 1993, 1994, 1995, 1997, 2000, 2001, 2002, and 2009 due to the high water extractions for public supply and for irrigation practices. Two scenarios were studied as possible strategies for groundwater management in the Salinas area involving the irrigation canals and assuming that its development is feasible: 1) increase surface water imports from the canals to farms and decrease or eliminate groundwater extractions for irrigation practices, 2) create infiltration ponds that use collected runoff, treated wastewater and exceeding water from irrigation canal as source of artificial aquifer recharge in strategic areas. Mitigation on groundwater depletion in Salinas could be planned in such a way in which 75 percent of the irrigation water requirement is provided by the irrigation canals and 25 percent by pumpage. On the other hand, it was found that the use of 2 shallow infiltration ponds of a total volume of 12,335 m3 that are constantly feeding the aquifer with percolated water produce positive net groundwater recharge even in the worst case scenarios and could increase the average net recharge from 64 to 236 mm/year.