USDA - ARSInvestigator
Donald Suarez, Ph.D.
US Salinity Laboratory, ARS-USDA
Corwin, D., and Goldberg, S.Summary
This report summarizes a statistical analysis of the of the relationship between marketable lettuce yield, soil salinity, apparent field-average leaching fractions and the depth to water table across the South Gila and Yuma CWUA Water Districts. The data presented in this report are based on a stratified random sampling of 19 lettuce fields throughout these two districts and include detailed measurements of marketable lettuce yield (Iceburg and Romaine varieties), soil salinity, leaching fractions, and field average depth to water table estimates. This analysis has been performed as part of the ARS research agreement # 5310-13610-013-15S, an inter-agency agreement between the U.S. Bureau of Reclamation (Yuma Area Office), the Yuma Agricultural Center (University of Arizona), and the USDA-ARS George E. Brown Jr. Salinity Laboratory to provide a statistical analysis of the expected economic damages (with respect to lettuce production) due to soil salinity, leaching practices, and/or shallow water table conditions throughout these districts. The statistical analyses presented here indicate that on average, marketable Iceburg and/or Romaine lettuce yields decrease by 4.77 Mg/ha per unit increase in soil salinity (ECe, dS/m, weighted 0-60 cm depth), once the soil salinity exceeds about 0.7 dS/m. Note that this estimated salt-threshold value is 0.6 dS/m lower than (and statistically different from) the 1.3 dS/m threshold level typically reported in the crop science literature (Shannon & Grieve; 1999). Additionally, these analyses suggest that both under- and over-leaching decreases the field average marketable yields and that excessive over-leaching practices appear to be more problematic throughout the two districts. For example, in a typical over-leached field within the S. Gila or Yuma CWUA districts, about 72 % of the total estimated yield loss appears to be due to excessive leaching. None of the analyses presented here indicate that the water table level effects either the field average or site specific salinity level(s), leaching fraction(s), and/or marketable yield(s), provided that the depth of the water table > 7.5 ft. On average, the expected yield loss in a typical S. Gila or Yuma lettuce field is calculated to be 10.14 Mg/ha, with an approximate 95% confidence interval of 4.0 Mg/ha to 16.3 Mg/ha. We estimate that about 6.58 Mg/ha of this expected loss is due to sub-optimal leaching practices (primarily over-leaching), while the remaining 3.56 Mg/ha loss can be attributed to excessive salinity levels. Assuming a $406 gross dollar value figure (for 1 Mg of lettuce) and the reported 14,449 ha area production estimate, the total dollar loss estimate across these two districts during the 2004 lettuce growing season was 59.5 million dollars (with an approximate 95% confidence interval of 23.6 to 95.4 million dollars).