IRRIGATION MANAGEMENT TO OPTIMIZE
CONTROLLED DRAINAGE IN A SEMI-ARID AREA[1]
Dr. Ir. R.W.O. Soppe[2],[3], Dr. J.E. Ayars2, Dr. E.W. Christen[4] and Dr. P.J. Shouse[5]
ABSTRACT
On the west side of the San Joaquin Valley, California, groundwater tables have risen after several decades of irrigation. A regional semi-permeable layer at 100 m depth (Corcoran Clay) combined with over-irrigation and leaching is the major cause of the groundwater rise. Subsurface drain systems were installed from the 60’s to the 80’s to remove excess water and maintain an aerated root zone. However, drainage water resulting from these subsurface systems contained trace elements like selenium, which were determined at toxic levels to fish and waterfowl. To maintain healthy levels of salt and selenium in the San Joaquin River, the natural drain out of the San Joaquin Valley, outflow of drainage water from farms was severely restricted or completely eliminated. Several on-farm management methods are being investigated to maintain agricultural production without off-farm drainage. One method is drainage water reuse through blending with irrigation water. Another method is to reuse drainage water consecutively, where drainage water from one field is used as irrigation water for another field. Progressively more salt tolerant crops need to be grown in such a system along the reuse path, and salts can eventually be harvested using solar evaporators. A method described in this paper aims to reduce the volume of drainage water during the growing season by increasing shallow groundwater use by crops before it is drained from the field.
Five years of crops were grown on two weighing lysimeters using drip irrigation. Two years of cotton were grown under high frequency drip irrigation (applications up to 10 times a day), followed by two years of safflower (early season crop) and one year of alfalfa (perennial) under low frequency drip irrigation (twice a week). One lysimeter maintained a shallow groundwater table at 1.0-m below soil surface, while the other lysimeter was freely drained at the bottom (3.0-m below soil surface). High frequency irrigation requires more irrigation water over a season than low frequency irrigation in the presence of shallow groundwater, since low frequency irrigation induces more shallow groundwater use by crops. Groundwater use for cotton was measured as 8% of total seasonal crop water use, while measurements under safflower showed that 25% of seasonal crop water use came from groundwater. Measurements under alfalfa, in its first year of establishment, showed 15% of seasonal crop water use coming from the groundwater.
To maintain a sustainable system, leaching of salts need to occur. Leaching under the proposed irrigation/drainage management system would occur in the early growing season with winter precipitation, pre-plant irrigation and the first irrigation of the growing season, when the water table can be maintained at shallower depths through restriction of the outflow of the subsurface drainage system (groundwater control).
Keywords: Shallow Groundwater Use, Controlled Drainage, Irrigation Management, Crop Selection, Soil Salinity.
1 Introduction
The San Joaquin Valley in California is an important agricultural area for crop production. The Valley is bound by the coastal range in the west, the Sierra Nevada in the east, the Tehachapi mountains in the south, and the San Francisco Bay-Delta in the north. Most of the Valley is divided into an east and a west side by the San Joaquin River.
On the east of the river, soils are generally light and orchards and vineyards can be found there. On the west, soils are generally heavy, and mainly row crops are grown there. Average reference evapo-transpiration rates in the valley vary between 1-2 mm/day during the winter months (Nov-Feb) and 7-8 mm/day during the summer months (Jun-Aug). On average, the rain falls during the winter months (summer precipitation is generally negligible for crop production), and average annual precipitation is approximately 300 mm. In general, more rain falls in the north of the valley than in the south, and less rain falls in the west than in the east of the valley.
From the 30’s to the 70’s, large irrigation systems were developed, importing water from north of Sacramento to the San Joaquin Valley, and from constructed reservoirs in the Sierra Nevada. On the west side of the San Joaquin Valley groundwater has risen after several decades of irrigation. A regional semi-permeable layer at 100 m depth (Corcoran Clay) combined with over-irrigation and leaching is the major cause of the groundwater rise. With shallow groundwater, salinity is a problem as well. Salts, naturally occurring in the soil, as well as imported into the Valley with irrigation water accumulate in the root zone and must be leached to maintain crop production levels.
Subsurface drain systems were installed from the 60’s to the 80’s to remove excess water and salts, and maintain an aerated root zone. However, drainage water resulting from these subsurface systems contained trace elements like selenium. High concentrations of selenium were determined at toxic levels to fish and waterfowl in drainage water collection ponds, especially the Kesterson Reservoir. To maintain ecologically healthy levels of salt and selenium in the San Joaquin River, the natural drain out of the San Joaquin Valley, outflow of drainage water from farms was severely restricted or completely eliminated, and collection ponds were closed and reclaimed.
Several on-farm management methods are being investigated to maintain agricultural production without off-farm drainage. One method is drainage water reuse through blending with irrigation water. Another method is to reuse drainage water consecutively, where drainage water from one field is used as irrigation water for another field. Progressively more salt tolerant crops need to be grown in such a system along the reuse path, and salts can eventually be harvested using solar evaporators.
Another method, discussed further in this paper, is through the use of controlled drainage. Controlled drainage usually restricts the outflow of drainage, either partial or complete, for periods of time when drainage water production is high. This will result in a higher water table in the field. The shallower groundwater can be seen as a source of water to the crop.
2 Methods and materials
2.1 Weighing lysimeters
Two weighing lysimeters were installed at the research farm of the USDA in Parlier, California. The lysimeters were each 4-m x 2-m on the surface and 3-m deep. The top 2-m of the soil profile was taken as an undisturbed soil monolith from the west side of the Valley, approximately 15 km south of the town of Mendota. The soil type was classified as an Oxalis silty clay loam soil (Harradine et al. 1956). The soil consists of approximately 30% clay, 55% silt and 15% sand. The salinity of the soil profile (ECe) was approximately 5 dS/m over the root zone.
One of the two lysimeters was free draining at the bottom (RefTank), while in the other lysimeters a water table was maintained at 0.9-m depth (GWTank). The salinity of the groundwater was stable over the five years of this study between 14 and 16 dS/m.
Over a period of 5 years, 3 different crops were grown. The first two years, cotton was grown, followed by two years of safflower, and the fifth year was an establishing year for alfalfa. Drip irrigation management was different for the cotton crop than for the following safflower and alfalfa crop. The cotton was irrigated using high frequency (up to 10 applications a day) subsurface drip irrigation, while the other crops were grown using low frequency (2 applications a week) surface drip irrigation.
A Mariotte bottle maintained the groundwater level in the GWTank at a constant level. However, several times during peak demand in the summer, the water table measurements showed a decline of the groundwater level, thus indicating that the Mariotte bottle could not keep up with groundwater use. The level in the Mariotte bottle was measured hourly, thus hourly contributions to the groundwater were measured.
The weight of the lysimeters was recorded hourly. Groundwater levels were initially measured manually, approximately once a week, while later an automated inverted PVC cup and a pressure transducer were used to automatically record the groundwater level hourly.
2.2 Field Research
During a four-year period, two field plots where controlled drainage was applied were studied. In one field (BV), the land user managed all farming practices, including irrigation. The other field (WL) both the drainage control structure and the irrigation scheduling were managed specifically for research.
The field called BV was located approximately 10 km west of Firebaugh, CA. The drainage system in the field was installed at 2.0-m below field level, and the tiles were installed 95 m apart. Tile drains were installed along the elevation lines, with the collector drain perpendicular to the elevation lines. The field under one drainage system was 120 ha, with the field divided in two equal sections of 60 ha (Fld01 and Fld03) for farming purposes. Soil type was a Ciervo clay (Fine, smectitic, thermic Haplocambids) (SSURGO, 2002), which exists of 50% clay, 30% silt and 20% sand. Cropping pattern over five years is shown in Table 1. Irrigation was applied during the season through furrow irrigation, but often pre-plant irrigation and the first irrigation of the season was applied using sprinkler sets. Furrows for irrigation purposes were not longer than 400 m.
Table 1: Cropping pattern on field BV from 1996-2002
|
|
Fld 01 |
Fld 03 |
|
1996 |
Cotton |
Melons |
|
1997 |
Cotton |
Cotton |
|
1998 |
Cotton |
Cotton |
|
1999 |
Alfalfa (new) |
Melons |
|
2000 |
Alfalfa (est) |
Cotton |
The field called WL was located approximately 5 km east of Kettleman City, CA. The soil type is classified as a Tulare Clay (Fine, montmorrillonitic, calcareous, thermic Vertic Haplaquoll) (USDA-SCS, 1986), which contains approximately 60% clay, 30% silt and 10% sand. The drainage system in this field was installed at 1.3-m depth with a distance between the tile drains of 28 m. Tile drains were also installed parallel to the elevation lines, and the collector line ran perpendicular to the elevation lines. The field under a single drainage system was 110 ha, and all three years (1996, 1997 and 1998) a cotton crop was grown. The whole field was managed as a single unit. The field was divided into basins of 75 m wide and 1600 m long for irrigation purposes. Irrigation water was applied through the use of a “sideboom”; a high-volume low-head pump mounted on a track-laying tractor that pumped water directly from a main canal into one of the basins, using a flow rate of 1.0 m3/s. Irrigation was terminated once water reached 2/3rd the length of the field (1100 m), and after 24 hour, the water still remaining in the field was surface drained. The field was sampled three times between 1996 and 1998. Each time, the same 42 locations were sampled in 30 cm intervals to a depth of 120 cm.
Collector drains at the lowest point in each field were excavated, and a manhole was located in the line, thus making access to drainage water possible. At both locations, a structure was inserted that allowed for control of drainage water outflow. The level at which drainage water flow would occur could be manually adjusted by adjusting the height of the structure (Schoneman and Ayars, 2000).
3 Results AND DISCUSSIOn
3.1 Groundwater Use by Crop
Measurements in the lysimeters showed that groundwater use for cotton was 8% of total seasonal crop water use (Soppe and Ayars, 2000), while measurements under safflower showed that 25% of seasonal crop water use came from groundwater (Soppe and Ayars, 2003). Measurements under alfalfa, in its first year of establishment, showed 15% of seasonal crop water use coming from the groundwater.
Groundwater contribution for cotton was well below the expected potential of the crop to extract water from groundwater. The most important reason for this low contribution was irrigation management. Since high frequency irrigation was used (water application several times a day) and the available water to the plant was kept constant, most of the active root water uptake was around the drip emitters (Soppe et al, 2002).
Over the next two years, when safflower was grown, irrigation scheduling was adjusted to a water application twice a week. Not only did this increase the fluctuation in total available water in the root zone on a weekly basis, but during peak water demand in the season it also depleted stored soil water, since the total irrigation application was less than the crop water use between irrigation applications. This was a result of the capacity of the storage tanks for irrigation water, but did not result in visual stress of the crop, or apparent yield loss.
The same irrigation schedule was applied on the establishing alfalfa. However, this crop did show visual signs of water stress, and irrigation had to start early in the season, maintaining the total available water in the root zone higher than the safflower crop. This resulted in lower groundwater contribution to crop water use than when the safflower was grown.
3.2 Drain Water Volume
Figure 1 shows the drainage outflow from field WL for three growing seasons. No controlled drainage was applied in 1996, the initial year of the project, while in the subsequent two years, drainage water outflow was restricted. Note how the first irrigation of the season produces the largest volume of drainage water, while the last irrigation of the season creates the least amount of drainage water. Reduction of drainage water volume in 1997 and 1998 was a result of less irrigation applications. Irrigation was scheduled in 1997 and 1998 on a combination of leaf water potential and soil water deficit, while in 1996 irrigation was scheduled based only on leaf water potential. Applied irrigation depths are shown in Table 2. Scheduling based on soil water deficit in the root zone automatically incorporates shallow groundwater use by the crop (Ayars and Soppe, 2002).
Table 2: Applied depth of water during irrigation season for field WL
|
Year |
Applied seasonal irrigation depth |
|
1996 |
750 mm |
|
1997 |
330 mm |
|
1998 |
220 mm |
|
1999 |
390 mm |
Although drainage water production was lower in growing seasons 1997 and 1998, Figure 2 shows that most drainage water was produced outside the growing season. This is the result of a combination of precipitation and pre-plant irrigation. Pre-plant irrigation is needed on this clay soil to facilitate land preparation. Without pre-plant irrigation, it is not possible to obtain the correct water content for a good seedbed.
Figure 1: Seasonal drainage flow (June-Sept) for field WL from 1996-1998
Figure 2: Cumulative 3-year drainage flow for field WL from 1996-1998. Symbols indicate June 1 of each year, usually when the first irrigation is applied. Dashed line is estimated drainage flow during winter period.
In field BV, drainage water outflow over the three years has a more constant flow (Figure 3). In 1996, half of the drainage area was cultivated with melons, requiring a different irrigation application, resulting in a lower drainage outflow than in 1997 and 1998.
Drainage water production in field BV is much higher than drainage water production in field WL. This is a result of several differences between the fields. The drains are installed at a greater depth in BV than in WL, regional groundwater flow is higher in BV (thus, more interception) than in WL, the soil has a higher conductivity and lower water holding capacity in BV than in WL, thus resulting in higher irrigation frequency and higher irrigation losses in BV. Seasonal applications for field BV are shown in Table 3.
Controlled drainage in field BV does not seem to affect the volume of drainage water. This is a result of irrigation scheduling, which was not integrated with drainage management during the growing season. For controlled drainage to have a positive effect on the overall water management on the field, irrigation management needs to be adjusted at the same time that active drainage management is practiced. The salinity of drainage water between the years does not show large fluctuations, and follows measurements by Ayars and Meek (19xx).
Figure 3: Seasonal drainage flow (June-Oct) for field BV from 1996-2000
Table 3: Applied depth of water during irrigation season for field BV
|
Year |
Applied seasonal irrigation depth |
|
1996 |
409 mm |
|
1997 |
526 mm |
|
1998 |
480 mm |
|
1999 |
364 mm |
|
2000 |
449 mm |
Figure 4 shows a different cumulative annual drainage flow for field BV than was shown for field WL. Most drainage water is produced during the growing season, while the winter period shows no or low flow. The seasonal drain water flow shows two increases: The start of pre-plant irrigation, between February and April, and the start of the seasonal irrigation, between the end of June and the beginning of July. The low flow periods during the winter are in part the result of controlled drainage, namely turning off the sump pumps. The water district that this field is part of has restrictions on the salt and selenium load that they can dispose of outside the district. To prevent exceeding these limits, drainage pumps are sometimes turned off when no crop is grown on the field. Another reason is that since irrigation halted, the regional groundwater levels decreased, and less regional groundwater was intercepted by the drains.
3.3 Salinity profile
Adjusted irrigation management (reduction of application) combined with controlled drainage in a saline environment must be adjusted for root zone salinity control as well. Figure 5 shows that in May 1997 the total salt mass in the root zone for field WL was higher than in subsequent years. Soil salinity measurements in 1998 and 1999 (both years had controlled drainage) show less salt mass, despite small seasonal irrigation applications (Table 2).
No major salt mass fluctuations were expected for field BV, since irrigation management had not been adjusted to controlled drainage. Figure 6 shows the salt mass distribution in the root zone for field BV. The lowest salt mass was measured in April 2000. This follows a year with a high drainage outflow (Figure 3) but low seasonal irrigation applications (Table 3), indicating high off-season leaching out of the root zone. This leached salt is not removed from the field (winter drainage practically zero (Figure 4)) but stored below the root zone.
Figure 4: Cumulative 5-year drainage volume for field BV from 1996-1998
Figure 5: Average mass of salt for three sampling times for field WL. Each point represents 42 measurement.
Growing season 2000 shows a low total drainage outflow for the season and an increase in salt mass in the root zone. This was the first full year of established alfalfa on half of field BV. The difference between an establishing alfalfa crop and an established alfalfa crop is that the root system of an established crop is already fully developed. The potential of shallow groundwater use is therefore larger than for an establishing crop. The reason that salt mass does not increase largely compared to seasons with an establishing crop could be that groundwater is used at the bottom of the root zone (as suggested in Soppe and Ayars, 2003), where salts will accumulate, but not at shallower depths in the root zone. The salts at the bottom of the root zone can be leached fairly easy since regional groundwater fluctuation allows for high mobility of the salts.
Figure 6: Average mass of salt for seven sampling times for field BV. Each point represents 17 measurements.
4 Conclusions
Irrigation management can induce crops to use larger amounts of groundwater for their water demand. However, irrigation management is limited by crop tolerance for water and salt stress. Keeping available soil water in the root zone high will reduce groundwater uptake by a crop. Larger irrigation intervals, or lower irrigation applications will increase groundwater uptake.
Controlled drainage can reduce drainage volume if it occurs together with adjusted irrigation management to increase crop water use from the groundwater. Controlled drainage without adjusted irrigation management does not necessarily result in reduced drainage water volume. Controlled drainage is a good tool to focus attention on irrigation management.
The effect of controlled drainage during the season might be small if large drainage volumes are produced during the off-season, as is the case for field WL. Controlled drainage during the off-season (in field BV through turning off the drainage pumps) does not necessarily result in an increase in salinity in the root zone.
Increased groundwater use during the season does not necessarily result in long-term salinization of the root zone. Off-season leaching appears enough to maintain a root zone salinity sufficient to grow a crop.
It is likely that the described systems are not long-term sustainable without removal of salt from the root zone – groundwater system (for example through salt harvesting). The storage capacity of the groundwater for salts is limited. However, comparing the volume of salt stored in the root zone and groundwater with the volume of salt added by irrigation water and removed from the system by drainage water shows that it will take a long time (several hundreds of years) before the storage capacity of groundwater is exceeded.
5 REFERENCES
Ayars, J. E., and D. W. Meek, 1994. Drainage load-flow relationships in arid irrigated areas, Trans. Of ASAE, 37(2), pp 431-437.
Ayars, J.E., and R.W. Soppe, 2002. Irrigation scheduling of cotton with capacitance probes in the presence of shallow saline ground water. In: Proceedings of the First International Symposium on Soil Water Measurement using Capacitance, Impedance and Time Domain Transmission (TDT). Held at the USDA-ARS Beltsville Agricultural Research Center, in Beltsville, Maryland, November 6, 7, and 8, 2002.
Harradine, F.F., Gardner, R.A., Rooke, L.G., and Knecht, E.A., 1956. Soil Survey of the Mendota area, California Series 1940, US Department of Agriculture & University of California Agricultural ExperimentStation
Schoneman, R.A. and J.E. Ayars, 1999. Continuous measurement of drainage discharge. Appl. Eng. in Agr., 15(5), pp. 435-439.
Soppe, R.W.O., J.E. Ayars. 2000. Characterizing groundwater use by cotton using weighing lysimeters. Proceedings of ASAE National Irrigation Symposium, June, Phoenix, AZ.
Soppe, R.W., J.E. Ayars and M.E. Grismer, 2002. Using capacitance probes to measure soil water in lysimeters with shallow saline ground water. In: Proceedings of the First International Symposium on Soil Water Measurement using Capacitance, Impedance and Time Domain Transmission (TDT). Held at the USDA-ARS Beltsville Agricultural Research Center, in Beltsville, Maryland, November 6, 7, and 8, 2002.
Soppe, R.W.O. and J.E. Ayars. 2003. Characterizing ground water use by safflower using weighing lysimeters. Agricultural Water Management, 60(1): 59-71.
SSURGO, 2002. http://www.ftw.nrcs.usda.gov/ssur_data.html. Last visited 7/28/03
USDA-SCS, 1986. Soil Survey of Kings County, CA