Lysimeter Project in Rocky Ford

One of the recommendations that came out of the Kansas v. Colorado Arkansas River Compact litigation is for Colorado to use the ASCE (American Society of Civil Engineers) Standardized Penman-Monteith equation to estimate crop consumptive use in the Arkansas River Valley. The Penman-Monteith equation (PME) calculates the evapotranspiration (ET) of a reference crop, which in Colorado is alfalfa, using meteorological data such as maximum and minimum temperature, relative humidity, solar radiation, and wind speed (Allen et al., 1998). The ET of other crops (ETc) is derived from reference ET (ETr) with the equation:

ETc = ETr x Kc for well-watered crops.

Kc or crop coefficient varies with crop type, growth stage, crop condition (plant density, health, etc.), and soil wetness, among other things. When the crop is water-stressed, ETc = ETr x Kc x Ks. The coefficient Ks is derived from the water balance (water inputs minus water outputs) in the root zone.

ETr is defined as the evapotranspiration of a non-stressed, well watered alfalfa crop, 50 cm in height, covering the ground fully. In other states, the reference ET is that of a non-stressed grass or similar short crop that is 12 cm in height at full canopy and is usually denoted ETo. Direct measurement of ET is best achieved with weighing lysimeters. Precision weighing lysimeters measure water loss from a control volume by the change in mass with an accuracy of a few hundredths of a millimeter. Non-weighing lysimeters are more common but they “are not considered suitable for reference ET equation verification and crop coefficient research. They may, however, be very suitable low cost alternatives for studying the effects of varying water salinity levels and high water table conditions on crop ET up and down the Arkansas River Valley.” (Ley, 2003).

In the absence of locally generated algorithms for calculating ETr with PME and Kc, the Colorado Division of Water Resources (DWR) has been using estimates from Kimberly, ID and Bushland, TX. However, the crop growing conditions (soil, elevation, climate, etc.) in the Arkansas Valley vary greatly from the prevailing conditions in Kimberly or Bushland. In his findings relating to the Arkansas River Compact compliance litigation initiated by Kansas, Special Master Arthur Littleworth accepted that the method used for calculating crop consumptive use in the Arkansas Valley be changed from Blaney-Criddle to PME. Consequently, Colorado’s Attorney General requested that the Colorado Water Conservation Board (CWCB) fund the “design, installation, and operation of weighing lysimeters at the Colorado State University Agricultural Experiment Station at Rocky Ford, Colorado”. The requested funds also cover the enhancement of CoAgMet weather stations, the investigation of irrigation water management in the Arkansas Valley, and the review of the changes made to the Hydrological-Institutional (H-I) Model by experts. The H-I Model has been used by the State Engineer’s Office (DWR) to determine depletions to usable water flows to Kansas.

Colorado State University (CSU) has a network of twelve automated weather stations along the Arkansas Valley. Temperature, solar radiation, humidity, and wind speed data from these stations will be used to validate ETr and Kc estimates for the whole Valley.

The lysimeter project at the Arkansas Valley Research Center (AVRC) consists of one large weighing lysimeter and one reference lysimeter. The large or test lysimeter was installed in 2006 and the reference lysimeter will be installed in 2008.

The project objectives, according to Thomas Ley of DWR (2003), are to:

Evaluate the performance and predictive accuracy of the ASCE Standardized PME for computing alfalfa reference crop ET for the growing conditions in southeastern Colorado,
Determine crop coefficients (for use with PME) for the various crops grown in the Arkansas River Valley under well-watered conditions, and,
Determine the effects of typical local growing conditions (which may include limited irrigation, high water table conditions and irrigation with water of high salinity contents) on crop water use.

The latter objective may require additional lysimeters e.g., non-weighing ones to achieve. It is worth noting that the effects of limited irrigation, high water table, and salinity on crop growth and water use in the Arkansas Valley have been studied by CSU scientists for several years using traditional (water balance estimates) and non traditional (remote sensing) methods. However, the impact of salinity for example on crop water use can be determined more accurately with a weighing lysimeter. Relatively high salt levels have been reported in the soils and waters of the Arkansas Valley (Gates et al., 2006).

The installation of the test lysimeter was completed in the fall of 2006, but some of the meteorological sensors were put in place in 2007. Consequently, it will be two to three years before achieving objective no. 1 and several more years before having usable Kc values and formulas for the major crops grown in the Arkansas Valley.

In the remainder of article, we will describe the main characteristics of the test lysimeter and its location and briefly review land preparation, crop establishment, and future plans.

Site characteristics:

The lysimeter is located at the Arkansas Valley Research Center, approximately two miles east of Rocky Ford in Otero County, Colorado (NW1/4 Sec 21, T23S, R 56W). The elevation at the site is approximately 1,274 m, latitude: 38 o 2′ 17.30″, and longitude: 103 o 41′ 17.60″. The soil type is Rocky Ford; coarse-loamy, mixed, superactive, mesic Ardic Argiustoll. Selected soil properties are shown in Tables 1 and 2.

Table 1. Soil characteristics of the test lysimeter site.

Horizon	Depth (cm)	Textural class	pH water (1:1)	CEC (meq/100 g)	ECe (dS/m)	Total C g/kg	SAR
Ap	0-23	Clay loam	8.1	17.2	0.82	15.5	1.70
Bt	23-36	Clay	8.0	16.9	0.90	14.8	2.08
Btk	36-100	Loam	8.3	10.0	0.58	9.0	2.46
Bk1	100-170	Loam	8.3	10.9	0.72	9.5	2.40
Bk2	170-230	Clay loam	8.3	13.5	0.88	10.8	2.18
2C	> 230	Course sand	8.7	1.5	-	1.7	-

Table 2. Soil bulk density and hydraulic properties (calculated)

Horizon	Depth (cm)	Bulk density (g/cm 3)	Matric suction in J/kg						Hydraulic conductivity (cm/hr)
Horizon	Depth (cm)	Bulk density (g/cm 3)	1500	1000	500	100	33	10	Hydraulic conductivity (cm/hr)
Ap	0-23	1.36	123	131	144	182	214	254	0.34
Bt	23-36	1.36	124	132	145	182	213	252	0.33
Btk	36-100	1.45	77	84	97	134	167	213	1.25
Bk1	100-170	1.43	82	89	103	141	176	224	1.06
Bk2	170-230	1.35	118	126	141	183	219	266	0.42
2C	> 230	1.86	19	22	26	40	53	73	16.9

The long-term average annual precipitation at the site is 11.8 inches, with May through August having the highest rainfall. The total average annual snowfall is 23.2 inches. The average minimum temperature is 36.3 oF and the average maximum temperature 70.0 oF. The last spring frost (32.5 oF) occurs on or before May 1 and the first fall frost on or before October 5 in 50% of the years; thus the average length of the growing season for warm-season crops like corn is 158 days.

Lysimeter characteristics:

The test lysimeter consists of an inner tank of 10 ft x 10 ft x 8 ft and an outer containment tank. The chamber between the two tanks houses the weighing mechanism, the drainage tanks, data loggers and has standing room for a half-dozen people (Fig. 3). The inner tank was filled with undisturbed soil (soil monolith) from the same field where the lysimeter is located (Fig. 1). Figure 2 shows the tank being lowered into its permanent location. The soil tank moves freely within the outer tank and the two are separated at the top by a fraction of an inch.

Figure 1. The inner tank being pushed into the ground to acquire the soil monolith. Photo taken by Dale Straw of DWR.

The weighing mechanism consists of a mechanical lever scale-load cell combination. The load cells are connected to Campbell Scientific CR-7 data logger which records the weight of the inner tank plus soil every 10 seconds. The readings are given in millivolts per volt (mV/V). A thorough calibration procedure was performed in 2006 to convert the load cell output in mV/V to the weight of water in kilograms. The standard deviation of the weight measurements (accuracy) was less than 0.02%. The change in total weight of the soil tank represents the amount of consumptive water use (transpiration plus evaporation from the surface of the soil monolith) by the crop. An example of load cell reading is shown in Figure 4.

Figure 3. Inside the containment tank (west side). Photo taken by Dale Straw of DWR.

Figure 2. The inner tank plus soil being lowered inside the containment tank. Photo taken by Michael Bartolo.

Water that percolates through the soil monolith is collected in two drainage tanks suspended from the scale frame that supports the soil tank, so that there is no overall weight change as water drains into the tanks. One tank collects water from the internal portion of the monolith and the other tank collects water from the perimeter of the monolith.

Instrumentation:

Several instruments are located in, above, or outside the monolith. They are used to measure:

Precipitation, wind speed and direction, minimum and maximum air temperature, barometric pressure, dew point temperature, relative humidity, and net radiation.
Incoming (from the sun) and reflected (from the ground or plants) radiation, and incoming and reflected photosynthetic active radiation (PAR)
Crop canopy temperature
Soil temperature at various depths and heat flux in or out of root zone
Soil moisture at 0- to 2.0 m in 20-cm increments with the CPN 503DR neutron probe. A calibration was performed to convert the probe readings into volumetric water content. The calibration procedure and results will be published elsewhere. Comparison of the soil water content inside and outside the soil monolith will be used to adjust the amount of water applied to the monolith and the amount of drainage.

Soil preparation

Shortly after the installation of the test lysimeter in 2006, the ground around it was flooded to settle the soil. Later, the ground was ripped with a Big Ox chisel plow to alleviate compaction, then plowed, disked, leveled, furrowed, and rolled. The distance between furrows is 30 inches, as is common in the Arkansas Valley. The top eight inches of the monolith were tilled with a roto tiller and the beds and furrows were prepared with shovels and spades. There are three full beds in the middle and a half bed against the eastern and western edges of the monolith, and four furrows. They are aligned with the beds and furrows outside the monolith and run north-south.

Figure 5. View of the lysimeter and meteorological instrumentation in late June 2007. Photo taken by Michael Bartolo.

The total area designated for the test lysimeter to ensure a good fetch is 10 acres (520 ft x 840 ft), of which 6 acres were fallowed since 2005 and an adjacent 4 acres was in alfalfa since 2003. It was paramount to get all 10 acres managed uniformly, thus in early spring 2007, the area in alfalfa was sprayed with Roundup and the whole field was planted to oats on 5 April 2007 at 140 lb/acre. The oat crop inside and outside the monolith was irrigated four times and cut for hay on 25 June. Figure 5 shows the lysimeter after the oat was cut.

The hay was baled on 2 July and the bales removed shortly after that. Oat was chosen as the first crop to be planted after the installation of the test lysimeter because it is easy to grow and could be planted and harvested early, allowing enough time for soil preparation and the seeding and establishment of the next crop (alfalfa) before fall dormancy.

In the latter part of July, the soil in the lysimeter field was again ripped, disked, and leveled. Alfalfa variety’Genoa’ was seeded on 9 August at 19 lb/acre and the field was then furrowed and rolled. The soil inside the monolith was prepared and seeded by hand. The number and arrangement of beds and furrows was the same as with the oat crop. Two hundred pounds of 11-52-0 per acre were broadcast on top of the hay crop on 6 December.

Alfalfa establishment inside and outside the monolith was good to excellent, with the exception of a couple acres approximately 100 ft west of the lysimeter. In this area, alfalfa stand was spotty due to a heavy infestation of morning glory. The whole field was mowed with a brush hog on 27-28 September above the hay crop to suppress the taller weeds. That is when it became clear that approximately half of the area west of the lysimeter will have to be reseeded in the spring of 2008 to achieve a more uniform stand with the rest of the field. Alfalfa was irrigated on 17 August, 4 September, and 4 October. Water from the irrigation canal was dispensed to each furrow with a siphon.

Irrigation of the soil monolith:

The monolith was irrigated each time the surrounding area was. The amount of water applied was determined by subtracting the amount that flows (flow x duration) in and out of adjacent furrows, as measured by v-shaped furrow flumes. Water was pumped from the irrigation canal and applied to the monolith through a hose fitted with a flow meter and a valve. The furrows on the monolith were filled with water to simulate normal flood irrigation (Fig. 6).

Figure 6. Water being applied to the soil monolith. Photo taken by Michael Bartolo.

Future plans:

The reference lysimeter (5 ft x 5 ft x 8 ft) will be installed in 2008 in an adjacent field and seeded to alfalfa. The area of the test lysimeter field that has a poor alfalfa stand will be reseeded in the spring of 2008. Alfalfa in the test lysimeter field will be maintained for at least three more years to calibrate the PME. After that, the field will be planted to corn and other major crops in the Arkansas Valley (corn, wheat, sorghum, onions, etc.) to determine their crop coefficients. It will take at least two years of data per crop to generate reliable Kc estimates. Reference ET will be measured with the reference lysimeter after the results are tested and validated.

The lysimeter project is a joint effort between CWCB, DWR, and CSU. Support has also been provided by USDA-ARS engineers and scientists in Fort Collins, CO and Bushland, TX.

For more information about the lysimeter project at AVRC, please contact Lane Simmons at lane.simmons@colostate.edu or (719) 469-5559.

References:

Allen, R.G., L.S. Pereira, D. Raes, and M. Smith. 1998. Crop evapotranspiration: Guidelines for computing crop water requirements. FAO Irrigation drainage paper 56. Rome, Italy.

Gate, T.K., L.A. Garcia, and J.W. Labadie. 2006. Toward Optimal Water Management in Colorado’s Lower Arkansas River Valley: Monitoring and Modeling to Enhance Agriculture and Environment. Colorado Water Resource Research Institute Completion Report No. 206. Colorado Agricultural Experiment Station Technical Report TR06-10, Colorado State University, Ft. Collins.

Ley, T.W. 2003. Lysimeters for evapotranspiration research in the Arkansas River Valley. Colorado Division of Water Resources.

College of Agricultural Sciences 121 Shepardson, (Campus Delivery 1101) Colorado State University Fort Collins, CO 80523-1101 Phone: 970-491-6274, Fax: 970-491-4895 Our url: www.agsci.colostate.edu Comments: CAS_webmaster@mail.colostate.edu Apply to CSU | Disclaimer | Equal Opportunity | Privacy Updated May 2, 2008

Agriculture - Civilization's first technology, still adapting for the future.

Lysimeter Project in Rocky Ford

Agriculture - Civilization's
first technology, still adapting for the future.