Water analysis and applying a leaching fraction for saline conditions

Written by: David Doll (UCCE Merced) and Daniel Sonke (Sureharvest, Inc).As discussed previously, sodium and chloride build-up in soils can cause crop loss by stunting plant growth. While much of the Central Valley has access to high quality surface irrigation water through irrigation districts, many almond orchards around the state have irrigation sources of variable quality.The first step in managing salinity is to know the source of salts. Water sources should be analyzed to determine the suitability for irrigation. Measurements of electrical conductivity (EC), sodium, calcium, and magnesium concentrations (cations), chloride, carbonate, bicarbonate, and sulfate concentrations (anions), pH, boron, and nitrate-nitrogen should be made. Most of these are standard.  Testing should occur on a regular basis since aquifer quality can change over time.Once the data is received from the test, the data should be checked for accuracy. First, the combined totals of all of the cations and the combined totals of all of the anions should be equal. Exclude boron and nitrate-nitrogen from these calculations. Next, if the EC is 5 dS/m or less, check to see if the sum of the cations is equal to 10 times the value of the EC. If these numbers are close, but not exact, the test is of good quality with all measurements made. If the EC and sum of cations are equal, most likely one of the cations/anions were estimated by subtraction rather than direct measurement. In the case of questionable quality, re-run the sample. Waters with ECs between 5 and 20 dS/m should use a multiplication factor of 12 instead of 10.Guidelines for water quality have been established to help identify excess salinity in water supplies. Estimating a 15% leaching fraction and the use of peach rootstocks (Nemaguard), the following table should be used as a guide to evaluate waters for suitable for irrigation and their effect on yield:

Salinity Measurement		Percent of Full Yield Potential
		100	99-40	<40
Irrigation Water (dS/m)		<1.1	1.1-3.2	>3.2

Specific ions sourced from irrigation water can also build up and cause problems. This table provides critical levels of the specified ions sourced from irrigation water. Again, these values are for peach type rootstocks (i.e. Nemaguard).

		Degree of Restriction
		None	Increasing	Severe
Sodium (SAR)		<3.0	3.0-9.0	>9.0
Chloride (meq/l)		<4.0	4.0-10.0	>10.0
Boron (meq/l)		<0.5	0.5-3.0	>3.0

Orchards with a water source that can affect yields or have an increasing or severe degree of restriction need to take salts into account especially when using drip irrigation or micro-sprinklers. The salts that make the water saline will accumulate within the rooting zone of the tree unless extra water is applied. The amount of water needed to flush the salts beyond the root zone is called the ‘leaching fraction’ or ‘leaching factor.’ For more information about the effective use of leaching to move salts and under what conditions it is recommended, see the Almond Production Manual from the University of California.Calculating the amount to apply as a leaching fraction is dependent upon the salinity of the soil and the salinity of the irrigation water source. Using the information from the water analysis, the following equations can be used to determine the proper ‘leaching fraction’:

Here is an example of calculating the leaching fraction for an orchard with the following parameters: 2.33 net inches of water needs to be applied, the efficiency of the irrigation system is 80%, the soil EC is 4.0 dS/m, and the EC of the irrigation water is 2.0 dS/m:

Using this example, 3.27 inches of water would need to be applied to supply the needed 1.98 inches and maintain the soil EC at 4.0. The extra 0.80 inches is used to compensate for the irrigation system inefficiency and the leaching fraction. Watering less than 3.27 inches will lead to a buildup of salts and increase the salinity problem.Please note that although leaching fractions are useful in maintaining the current salinity levels, they may not be suitable for perennial crops due to the potential for root health issues and in-season cultural practices. Furthermore, the high water demand makes it challenging to reduce salt levels. In the situations with high salinity levels, dormant leaching programs should also be considered.Reference: Allan Fulton, James Oster, and Blaine Hanson. Chapter 5: Salinity Measurement. Pgs 29- 40. The Almond Production Manual. University of California Agirculture and Natural Resources, Publication 3364. 1996.