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Water Quality Data
SOIL MOISTURE STATUS & INSTRUMENTATION
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SALT AND IRRIGATED AGRICULTURE
All of the ideas discussed in this chapter are affected by the amount and relative balance of salts in the soil and water. High or out of balanced salts in the soil can reduce effective available water holding capacities, restrict water infiltration, require more irrigations, and/or alter soil moisture tension readings, among other effects. The Westlands Water District is affected by high water tables in some areas and by high salts in groundwater in most areas. Please read the chapter on Salts and Drainage or see this link to a United Nations soil and water training manual that covers this same material, but describes things in metric units. .
SUMMARY
In summary, there is a body of science that has created a model of how water moves into the soil, through the soil, into the plant, and back out into the atmosphere.
We know that soil will only hold so much water, it does no good to irrigate over this maximum field capacity. We know that the water-holding forces in the soil go up as the level of soil moisture goes down. This leads to two ways of describing soil moisture- in terms of the actual water content (inches of water held per foot of soil), and in terms of the water-holding forces (centibars of tension).
We act in terms of the actual water content (inches of water per foot of soil), we need to know how much water to replace at an irrigation. The crop doesn't care how much water is in the soil. It cares how hard that water is being held by the soil (the soil moisture tension in centibars). Crops extract water at a measurable and predictable rate, evapotranspiration, ETc, measured in terms of inches/day. We do not let the crop use all the available water in the soil. As the soil moisture goes down, the forces holding water in the soil go up. It becomes harder for the plant to extract the water it needs.
We can describe the rate at which water enters the soil as an infiltration rate, measured in inches/hour. We can also describe the rate at which we apply water during an irrigation, the application rate, also measured in terms of inches/hour. If the application rate is greater than the infiltration rate, runoff will occur.
There are many ways of measuring water, both in terms of volume and in terms on tension. Each has its own strengths and weaknesses. Which is chosen depends on the specific situation.
The available water holding capacity of soils, the rate at which water soaks into and through soil, the level of stress on a plant at any soil moisture level, are all affected by excessive or imbalance salts in the soil. See other pages on Salinity for further information on these effects.
Last updated September 2000