Crop growth and water-use from saline water tables
How much water can a crop abstract from below a saline water table and how does the salinity affect yield? These questions are important because shallow groundwater may represent a substantial resource in flat, low-lying areas, but may also represent a threat to sustainability where salinity is high. A series of experiments in a glasshouse aimed to elucidate irrigation management practice under salinity conditions and to develop a root uptake model under both osmotic and matric stresses. The extraction of soil water and groundwater by lettuce and perennial ryegrass crops were measured in three instrumented lysimeters. Water table depths were 0.6,0.9 and 1.2 rn below the soil surface. The lysimeters were initially saturated with saline water (electrical conductivity 4.5 dS m- 1 for lettuce, 9.4 dS m- I for the first crop of ryegrass and 0.4,7.5 & 15.0 dS m-1 for the second crop of ryegrass) and drained until an equilibrium soil water profile was attained. Water with the same electrical conductivity was then supplied by Marione siphons to maintain the constant water table. The water table contribution was recorded and water losses from the soil profile were estimated from daily readings of soil water potential using tensiometersa; nd gypsum blocks. Solute samples were extracted periodically for salinity measurement. The cropping period of lettuce was 90 days from sowing and the lst & 2nd cropping periods of ryegrass were 223 & 215 days respectively. The first ryegrass experiment showed that the water table depth (60,90 and 120 cm) did not have significant contribution (37,36 and 36 mm) on either total soil moisture use or groundwater contribution. Similar results were found for total soil moisture use for lettuce, though the groundwater contribution varied significantly. The second ryegrass experiment showed that salinity at the water table strongly influenced total soil moisture use, but the total groundwater contribution varied only slightly. The overall crop experiments show that the groundwater contribution was within the range of 25-30% of the total water use, except for the 15 dS m7l treatment where the contribution was greater than the soil moisture use. Groundwater contribution rate was higher when the plants were subjected to more osmotic and matric stresses. Yield component data show that increasing salinity leads to a reduction in total yield, but the drymatter proportion was higher. Higher salinities occurred in the upper 15 cm of the root zone, because of the greater soil moisture depletion. Below that depth the salinization rate was smaller, because of the greater groundwater contribution in the later part of the season. There is reasonable agreement between measured and estimated (based on convective transport theory) values soil salinity. Salinities increased in the root zone by about 3-fold of initial salinity for lettuce and around 4-fold for ryegrass in the top 5 cm depth, but below 15 cm depth it was less than 2 fold. Finally, a simplified model was developed to describe the interaction of root-zone salinity and water uptake, considering salinity and water stress as additive. The model shows that the higher the root-zone salinity stress, the higher the predicted water uptake while plant uptake considered -1.5 MPa. This variation is ranged from 4 to 17% for 0.4 to 9.4 dS m-1 and 30 % for 15 dS m-1. The model was developed in a climate with low atmospheric demand, but needs testing in a more severe environment.