The growing population of the municipalities within Los Angeles and San Bernardino County high desert areas has placed an increasing demand on water resources as well as produced an abundance of municipal waste water. Many of the forage producers in the area are using ground water for irrigation because imported water is unaffordable. At the same time the growing municipal areas are creating a demand for water, especially ground water. Litigation between the city of Victorville and water users in the high desert area of San Bernardino County has resulted in ground water adjudication, limiting ground water use to forage growers. There is a great need for methods to conserve water in forage crop production in the region. An irrigation project has been funded on behalf of the SARE Program (Sustainable Agriculture Research Education Program) to be conducted with four growers in Los Angeles and San Bernardino Counties to monitor soil moisture under alfalfa irrigated with center pivots and onions under solid set sprinkler and drip irrigation.

Project 1 - 2003 - 2004 SARE Funded Irrigation Project

The SARE irrigation project consisted of the cooperation of three alfalfa growers and one onion grower. The project involved the installation of soil moisture monitoring equipment in alflalfa and onion fields to be used as devices for monitoring soil moisture to schedule crop irrigations.

The placement of the electrical resistance block sensors (Figure 1) is dependent upon the crop rooting depth. As a general rule, in vegetable row crops sensors are placed from 6 to 20 inches deep, the number and depth are dependent upon the crop. In tree fruits two sensors are often used, each placed at 18 inches and 36 inches deep. In alfalfa and pastures three sensors are commonly placed at 12, 24, and 48 inches deep. Shallower sensors are used to schedule irrigations while deeper sensors are used to monitor water percolating past the root zone.
Te sensor readings are expressed in centibars. The higher the number, the dryer the soil. The sensors are calibrated according to soil type and crop, as shown below in Table 1. Note: the critical values in Table 1 are for alfalfa and pasture. Note that care should be taken when using the blocks in sandy soils because the sensors may respond slowly to moisture conditions. Sensors should be carefully calibrated according to local conditions.
Centibar readings from the sensors can be collected by the data logging devices (Figure 2) and retrieved by downloading it to a personal PC, while the hand held digital meter (Figure 3) must be manually read on a weekly basis.

Table 1. Recommended values at which to irrigate alfalfa and pasture for different soil types.

Table 1 is adapted from Orloff et. al 2000. Orloff, S., B. Hanson, D. Putnam. 2000. Soil-Moisture Monitoring: A simple method to improve alfalfa and pasture irrigation management. University of California operative Extension, Siskiyou County, Yreka, CA. pgs. 2-5.
*Caution: Soil moisture sensors may not be useful for very sandy soils with extremely low water holding capacity, as the sensors may not respond quickly enough to the rapid decline in soil moisture.

The data from this project has been useful in showing the drying of soil moisture at the four foot depth in alfalfa during July and August under center pivot irrigation. Since center pivots do not put out a high volume of water with each application, it can be difficult to apply enough water to alfalfa during the high water use months in the summer. Many have recommended that it is very important to begin the season with a soil profile full of moisture to four feet in alfalfa. The data from this project suggests that there is significant drying to the four foot depth in the summer months, even when the soil profile has been filled with moisture during the winter and spring months (See Figure 1).

Figure 1. Soil moisture at the one, two and four foot depths from
January 1 to September 9, 2004 in an alfalfa field irrigated with a center
pivot near Lancaster, CA.

This same soil moisture monitoring technology was demonstrated in onions near Lancaster during 2004 (Figure 2). These onions were drip irrigated every three days. Notice the extreme drying at the 8 inch depth on the third day during the latter part of June and beginning of July. During mid July the grower started supplementing the drip irrigation with sprinkler irrigation to reduce the extreme drying at the 8 inch depth through July and August. The soil moisture remained stable at the 20-inch depth throughout the season, with the exception of late July/early August. There was some movement of water beyond the 20 inch sensor with a couple of irrigations during late July and early August, which represents over application. The grower cut back the water application during August to optimize irrigation applications and there was less drying at the 8 inch depth and less movement beyond the 20 inch sensor. This particular field yielded above average in onion yield and quality.

Figure 2. Soil moisture at the 8 inch and 20 inch depths from May through
September 2004 in an onion field under drip irrigation near Lancaster, CA.

2003 SARE Irrigation Management Field Days

Two irrigation field days were held in Los Angeles and San Bernardino Counties in August of 2003 to introduce new methods of soil moisture monitoring and techniques used to schedule crop irrigations. Blake Sanden, Kern County Farm Advisor made the meeting in Los Angeles County. At the field days there was a demonstration on the installation of the soil moisture monitoring equipment and representatives from several different irrigation companies were present to talk about irrigation management and equipment.

Blake Sanden, UC farm advisor, demonstrating the use of a hammer probe
to check soil moisture.

Installing electrical resistance blocks, or Watermark sensors in alfalfa

Figure 3. A small 7/8" diameter hole is made with a pipe or hammer probe.	Figure 4. The sensor is placed into the hole using this PVC pipe.
Figure 5. The sensor is placed in a soil/water slurry before it is placed in the soil to enhance sensor to soil contact.	Figure 6. When using a data logger, telephone wire can be shanked from the data logger location to the sensor site in the field, up to 1,000 feet.
Figure 7. It is best to use telephone wire between the sensors and data logger. The sensor wires are soldered and enclosed In this PVC case before being buried in the soil.	Figure 8. Demonstration of a tensiometer installation, another device used for soil measuring soil moisture.

Project 2 - The Application of Secondary Treated Effluent to Irrigated Forage Crops

The application of secondary treated municipal wastewater to irrigated agricultural crops has been widely accepted as a viable method of waste disposal (Pettygrove and Asano, 1984). Although there are many different types of applications of wastewater to irrigated crop land throughout the world, each case is unique due to variation in climate, soil type, water source, crop selection, crop market, and the irrigation method. This variation can have a dramatic affect on agricultural production. Due to an increase in the regulation of secondary treated wastewater applications to agricultural crops, there is a need to carefully monitor wastewater applications to agricultural lands to minimize the risk of groundwater pollution.

The growing population in the Antelope Valley has caused an increased demand for valuable groundwater resources as well as an overabundance of wastewater production. The disposal of secondary treated wastewater to irrigated crop land helps to alleviate these two issues by conserving groundwater that would other wise be applied to these crops while utilizing wastewater that has the potential to contaminate the groundwater if it is improperly disposed of. However, irrigating forage crops with wastewater requires more intense documentation of water application and crop yield than the normal forage grower would be accustomed to because of regulations and the potential risk of groundwater pollution. Wastewater should be applied to crops with the objective to maximize yields and minimize the leaching of water beyond the crop root zone through careful monitoring and application methods.

The determination of appropriate application rates is largely dependent on water and soil content (nutrients, salts, organisms), crop selection, the irrigation application method, crop water use (ET), and soil moisture monitoring techniques. In many wastewater sources nitrogen is the main constituent of concern because of its potential to contaminate the groundwater (Loehr et al., 1979). The main constituent of concern in wastewater applied to forage crops at the Los Angeles County Sanitation District reuse site near Palmdale is nitrogen. A project was conducted by the local farm advisor in cooperation with the Los Angeles County Sanitation Districts at the Palmdale wastewater re-use site near Palmdale, CA. during the 2003 growing season. Crop water use ET rates and soil moisture monitoring techniques were used to appropriately schedule crop irrigations and prevent wastewater over application and the potential for groundwater contamination.

Reference:

Pettygrove, G. Stuart, and Asano, Takashi, (eds.). 1985. Irrigation with Reclaimed
Municipal Wastewater - A Guidance Manual. Lewis Publishers, Inc., Chelsea, MI
pgs.10-14.

Loehr, R. C., W.W. Jewell, J.D. Novak, W.W., Clarkson, and G.S. Friedman. 1979. Land Application of Wastes, Volume I. Van Nostrand Reinhold Company, New York. pgs. 55-96.