University of California Water Resources Center

Twelve, 19.5 acre test plots arranged in a randomized complete block design are set within two 155 acre fields to provide a realistic production environment. These blocks of welldrained Panoche clay loam were formerly irrigated with California Aqueduct water and sprinklers for the last 30 years. Overall the field electrical conductivity (EC) ranged from 0.5 to 4.5, averaging 1.57 dS/m to a 3-foot depth. Saturation extract boron was 0.6 ppm. The area is underlain by a semi-saline aquifer that has been made worse over the decades by contamination from oilfield leachate water. Several production wells were drilled in fall 2003 to begin using this water. A drip tape irrigation system was set up to allow the planting of 6 rows of cotton every 22 feet the first year of the project (2004) followed with the planting of 1 year old pistachio seedling rootstocks March 2005 in 22 foot rows interplanted with 4, 38 inch rows of pima cotton. Salinity of the shallow groundwater for the test fields has varied from 4 to 7.5 dS/m with 8 to 11 ppm B. Three treatments were imposed: AQUEDUCT/CONTROL: EC ~ 0.4 dS/m (Aqueduct water only), BLEND: EC ~ 2.5 dS/m (50/50 mix) and WELL: EC ~ 5 dS/m.

Chloride and boron contents in late season cotton petioles and pistachio leaf tissues were significantly higher in the WELL treatment compared to the Control. Saturation extract EC and B in the top three feet of rootzone were significantly increased in the BLEND and WELL treatments over the Control at the end of the season, with most salts increasing significantly to 3 feet with a significant increase in Cl to 5 feet.

Cotton yield and tree development: Pima cotton lint yields were nearly 4 bale/acre in 2004, but crashed to about 2 bale/acre in 2005 due to very cool spring conditions that made for poor stand establishment. Cotton yields and plant height were unaffected by salinity. Spring 2006 provided excellent conditions for cotton growth, but excessive salts accumulated in the top 4 inches of the Well treatment beds reduced cotton emergence by 14% (statistically insignificant) and lint yield from the saline Well treatment was reduced by 275 lb/ac compared to the Aqueduct water. However, the Well treatment yield was still excellent at 3.12 bale/ac.

Increase in pistachio rootstock diameter and general tree development was unaffected by salinity for both rootstocks for the first three years, but PG1 rootstocks showed a significant 7% decrease at the end of 4th leaf (Figure 1). However, Photoshop® pixel counts of green foliage down the row show an equal amount of green foliage for all treatments as of 10/18/08.

Salinity and sustainability: At the end of 2006, after three seasons of cotton irrigation this program applied about 6,600, 32,500 and 54,000 lb/ac of salt in the Aqueduct, Blend and Well treatments, respectively. Rootzone salinity to 5 feet in the wettest part of the profile (between the two hoses) measured by Spring and Fall soil samples has remained surprisingly stable at about an ECe of 2.5 dS/m for the Aqueduct and about 5 to 6 dS/m for the Well treatment. However, in-season ECe in the top two feet is much higher as water and salts sub up from the buried drip tape. Without 6 to 10 inches of effective rainfall or fresh water winter irrigation for efficient leaching this system may not be sustainable. Due to the decrease in cotton yield in 2006, combined with a 50% increase in the Well water EC over the last four years we reduced the salinity of the Well treatment (by blending with Aqueduct water) down to 4.5 dS/m. This is about the salinity of the Well at the start of the test in 2004. After one year of cotton and four years of drip tape irrigation in the pistachios a total of 73,823 lb salt/wetted acre (about a 9.5 foot wide zone of subbing) was applied in the active rootzone of the Well treatment compared to 8,050 lb salt/wetted acre for Aqueduct water.

At an average pima price of $1.08/lb, an economic analysis of cotton production and yields for the year prior to and first two years after planting pistachios shows a net return of $2,120 for Aqueduct water @ $120/ac-ft and $2,249 for Well water @ $45/ac-ft for this system.