Abstract form

Salinity control under saline shallow ground water conditions of the
San Joaquin Valley, California
Blaine HANSON* 1 , Don MAY 1 , Jirka ŠIMŮNEK 2 , Jan HOPMANS 1
1 Dept. of Land, Air and Water Resources, University of California, Davis, CA, USA
2 Dept. of Environmental Sciences, University of California, Riverside, CA, USA
*Corresponding author’s email: brhanson@ucdavis.edu
Shallow saline ground water conditions have caused soil salinity problems along the
west side of the San Joaquin Valley, California. Subsurface drainage systems cannot be
used to control soil salinity and water table levels because even after more than 30 years
of research, no suitable method of drainage water disposal exists. Thus, improved
irrigation practices must be used for salinity control.
Research has shown that under furrow, border, and sprinkle irrigation, soil salinity near
the ground surface depends mainly on the salinity of the irrigation water, but soil
salinity at deeper depths increases as the ground water salinity increases. Soil salinity
increases during the crop season. A pre-plant irrigation in the early spring is used to
leach salts that accumulated during the previous year. About 25 mm of water per 30 cm
of soil depth in addition to that needed to replenish the soil moisture depletion are
needed to reduce the soil salinity and to prevent ground water intrusion into the root
zone.
Field studies have shown drip irrigation of processing tomatoes under shallow saline
ground water conditions to be highly profitable compared to furrow and sprinkle
irrigation, As a result, drip irrigation has increased in the saline soils of the valley.
These studies revealed that soil salinity in the soil profile depended on the amount of
applied irrigation water and its salinity and depth to the shallow ground water and its
salinity. Highly concentrated or localized leaching occurred near the drip line with the
zone of highly leached soil increasing as applied water increased. Yield also increased
as applied water increased due to both the larger volume of low salt soil and higher soil
moisture content near the drip line. A seasonal water application about equal to the
seasonal crop evapotranspiration provided sufficient localized leaching, yet prevented
intrusion of saline ground water into the root zone. However, it was found that the water
balance approach underestimated leaching fractions under drip irrigation.
The HYDRUS-2D computer simulation model revealed a leaching fraction of about
25% for a water application amount equal to seasonal evaporation, whereas the water
balance frequently showed no leaching. Simulated salt patterns around drip lines were
similar to those found in the field studies. Reclamation of drip-irrigated saline soil
occurred faster for relatively large applications applied two to three times per week
compared to smaller applications applied daily. The initial soil salinity conditions
affected the rate of reclamation.
Both field and simulation studies indicate that subsurface drainage systems and drainage
water disposal facilities may not be needed under properly managed drip irrigation.
Key words: soil salinity, drip irrigation, ground water
15