Irrigation Transect (WAT01)

PURPOSE     
    To assess the long-term response of selected vegetational and ecosystem parameters to annual burning with no water limitation.     

LOCATION OF SAMPLING     
    The transect and two control transects are located about 300 m (600 ft) southwest of the old stone reservoir and windmill to the west-southwest of headquarters (grid B-16).  The transect runs perpendicular to the slope just south of the belowground plots on the east side of the drainage way.  The area is identifiable by the line of sprinklers on 1 m risers from the 7.5 cm diameter aluminum pipe which runs a length of 140 m (460 ft) down the transect (Fig.35).  

    The area which is subject to additional water is outlined by a row of steel posts located 15 m (50 ft) on each side of the line of sprinklers.  Posts are located 18 m (60 ft) apart along the transect to correspond to every other sprinkler on the line. A numbering system is laid out in reference to the sprinklers.  The lower most sprinkler is 1 and the upper most sprinkler is 16. Every other sprinkler has a number mounted on its riser.  The steel posts have a number which corresponds to the number on the sprinkler it is nearest.  The total research area consists of the area inside the steel posts plus how much farther outside the potential wetted area the particular experiment needs to extend to get sufficient unirrigated area to compare to the irrigated area. As research areas outside the areas bounded by the steel posts is requested, individual researchers will be responsible to mark those areas with steel posts.

    Walking on the area is necessary to make measurements, get samples, etc.  When walking to a particular area, please walk either parallel or perpendicular to the sprinkler line and line of steel posts.  When walking up the transect or perpendicular to the transect, please walk on lines directly between steel posts or lines formed by steel posts and the closest sprinkler. Researchers may walk on their plots as necessary.  Walk only on the lines between steel posts and sprinklers to get to individual research areas.

    When an area is assigned to a researcher it will be located with reference to the nearest sprinkler, the irrigation line, and the row of steel posts.  You may mark it as you see fit.     

Species Composition Plots:     
    Species composition conduits are located east of each riser in the irrigated transect and in the eastern control transect. TDR probes for soil moisture measurements are located west of the sprinkler risers in the irrigation transect and in the western control transect.  Biomass is harvested on the west side of the risers away from the TDR probes, and reproductive effort is assessed on the eastern side of the irrigation transect and in the eastern control transect.  Leaves for plant xylem pressure potential measurements are collected from either side of the transect.     

FREQUENCY OF SAMPLING     

    Aboveground biomass is sampled in late August-September. Reproductive effort is sampled in October.  Plant water potential [predawn and midday (1300 CDT)] and soil moisture are sampled at ca. weekly to 10 day intervals depending on the weather. Species composition is sampled once each season in late June. Soil chemistry is sampled at five year intervals.     

VARIABLES MEASURED     
    1)   Aboveground biomass     
    2)   Plant reproductive effort     
    3)   Xylem pressure potential in Andropogon gerardii     
    4)   Plant species composition     
    5)   Soil moisture and chemistry      

METHODS

Irrigation Procedures:

General Information:

    A single line of 3/16-inch (4.76-mm) diameter nozzle, full-circle sprinklers with 30 psi pressure regulators below each one are spaced close (30 feet) together on the supply line to provide relatively uniform amount of water at locations parallel to the line. There is a gradient, in terms of water added, from most water at the line to none at the outer radius of the sprinklers (14 m from the line with no wind interference).  Fully-watered measurement areas are those 2 meters either side of the line.  The area out to 5 meters from the line receives 90% or more of the amount of water measured at 2 meters.

    Scheduling of irrigations is done according to the needs of the plants along the transect near the irrigation line. Soil water content is measured at two sites perpendicular to the transect and at six locations (2 m, 11 m, and 15 m from the pipeline) in the cross-section. The two locations, 2 meters either side of the line, on Transect B in the upper area are used for scheduling purposes. There, total water-holding content at field capacity in the 1.0-meter profile is 300 mm.  When the total water content falls to 250 mm, an irrigation is applied.  Irrigations are nominally about 25 mm, so some storage is left for subsequent rainfall. A spreadsheet is used for scheduling and it uses inputs of measured soil water content, daily effective rainfall, daily irrigation, daily reference evapotranspiration, and an estimated daily water-usage coefficient for each day of the growing season.  The water-usage coefficient is periodically revised during the season based upon the other parts of the water balance. Effective rainfall is the amount of water that will fill the soil profile back to 300 mm.  Additional rainfall is assumed to be lost by runoff or deep drainage.

    Nominal supplemental water needs for grass in the Manhattan area averages about 450 mm per year for cool season types. Warm season grasses require less because their active growing season is shorter then for cool season types. For this work, nominal average water addition has been 300 mm. Maximum water use occurs in early to mid July and averages about 6 mm per day for a fully-watered condition. So, irrigations as often as every four days may be needed during this period.

    A well at the reservoir provides about 12 gpm into the reservoir. An irrigation pump takes water from the reservoir and delivers it to the sprinklers at about 160 gpm. A nominal irrigation will be to run for four hours.  During irrigation with the well running, the reservoir drops about 1.2 feet per hour. So, the reservoir must have 5 feet of working depth when full, must be within about 0.5 feet of the top, before a four-hour irrigation can begin. A float switch on the well keeps the reservoir at about 5.25 feet of working depth.

Preparing for Irrigation:

    Irrigations should be scheduled when the wind speed is expected to say low, below 10 mph (15 kph) for the four-hour irrigation period. This condition most often occurs early in the morning.  Since irrigation can have problems such as plugged sprinklers, irrigating when it is daylight enough to observe the operation is necessary, especially when starting the irrigation.  Hourly wind conditions are forecasted for 48 hours in advance, so there is relatively good information available to aid with scheduling.

    The well will be started by the float switch when the irrigation pump begins to lower the reservoir level. The well will refill the reservoir in less than three days after it is emptied by an irrigation.

    A set of 24 water-catchment bottles (irrigages) within Transect A between Sprinklers 3 and 4 and the same setup on Transect B between Sprinklers 11 and 12 should be empty before beginning an irrigating.  The same goes for the irrigages located at the 6 locations on each of the transects where soil water content is measured.  The measurement made at these locations are assumed to represent the amount of water applied at other locations along the transects.  There are three irrigages in four rows perpendicular to the pipeline are at distances of 2 m, 7 m, 12 m, and 15 m.

Performing the Irrigation:

To carry out an irrigation do the following:

1) Empty the irrigages on the two transects.

2) Record the water depth reading in the reservoir to show how much water was removed from the reservoir.

3) Estimate and record the air temperature, wind direction, and wind speed on the data sheet.

4) Open the valve on the intake line to the pump (3-inch valve which is turned so that the handle is parallel to the intake line. Turn on the electricity to the pump, the switch in the control box located just above the supply line on the side of the reservoir.  The pump will run for about two minute before the sprinklers start to spray water. After about three minutes, all sprinklers should be operating fully.  Record the time all sprinklers are spraying water as the start time.  Check the pressure gage on the irrigation line to verify that is near 60 psi to insure that there are no leaks in the system.  A lower pressure will indicate a leak.

5) Monitor the operation of the system periodically.  Be particularly attentive to any plugged sprinklers or sprinklers that are not rotating.  Wait four hours until pump is ready to be shut off.  Check the water level in the reservoir.  If eddies develop about the intake system, it is time to shut off the irrigation pump regardless of the time it has been operating.  Shut off the irrigation pump and close the supply line valve by turning the handle up so that it is perpendicular to the supply line.   Record the time the pump is shut off as the ending time for the irrigation.

6) Again, record the water level in the reservoir, the air temperature, wind direction, and wind speed.

7) Using the portable balance, weight the bottle from each irrigage and record the weight. Empty the bottle and weigh it again for the tare weight and record the weight.  Do this for both sets of 24 irrigages and for the 6 irrigages at the soil-moisture measurement locations. Do the same for each of the two locations. If you anticipate rain before the next irrigation, put the empty bottle upside down in the top of its irrigage. This will keep it dry and minimize collecting insects in the irrigage.

8) Record any comments of interest or importance on the data sheet.

9) Return to the lab and enter the irrigation information into the appropriate spreadsheets to calculate the amount if irrigation across the transects and at the soil-moisture measurement locations.

Make sure that data sheet is returned to a designated place for safe keeping.

Plant Production Analysis:

Aboveground biomass:     
    Methods are identical to those in data set PAB01 except only four 0.1 m2 quadrats are harvested at each sampling point.  

Species composition:     
    Methods are those used for LTER species composition measurements (data set PVC02). Canopy coverage is estimated in one circular plot at each sampling point.     

Reproductive effort:     
    We estimate density of flowering culms of Andropogon gerardii, Sorghastrum nutans, and Andropogon scoparius by counting all reproductive culms in four randomly placed 0.25 m2 quadrats at each sampling point. Heights of reproductive culms are measured to the nearest cm by selecting the three nearest culms of each species at three randomly selected points at each sampling location (total of nine culms measured for each species at each sampling point). No harvesting occurs in this sampling scheme.

Plant xylem pressure potential:
    At least seven mature leaf blades are collected at each sampling location and immediately stored in a plastic bag with wet filter paper.  Leaves are transported back to the lab and xylem pressure potential is measured with a PMS Model 1000 Pressure Chamber to the nearest 0.1 MPa (1 bar). At the limestone break along the irrigation and control transect, leaves from a hackberry tree are also collected and measured.

Soil moisture:
    Soil moisture is measured at 0-15 and 0-30 cm depths with a Time Domain Reflectrometry System.