ELECCIÓN DEL RODAMIENTO A UTILIZAR

The proposed DSS was intended to assist decisions on the amount and timing of effluent irrigation by predicting the quantity and timing of future outflows (leachate volume and NO3-N concentration) from a LTS (Fig. 5.1). This DSS takes account of the soil’s ability to absorb the designed irrigation volume, the NO3-N concentration in the leachate, and the plant's ability to take up the applied amount of water and N.

In this DSS, the LEACHN model provides the predictions on which to base a management decision. Weather data, initial water and nutrient contents (SMC and SNC) of the soil profile, and the design effluent irrigation volume are the input parameters to the model. The model then predicts SMC and SNC, AET, plant uptake of nutrients, leachate volume and nutrient concentration (mg/L) in the leachate.

Fig. 5.1: A DSS to reduce the risk of groundwater contamination at a land treatment system.

To check the long-term effects of a proposed irrigation plan, the SMC and SNC on the starting day, along with a likely weather scenario and the proposed irrigation plan are given as the input parameters to the LEACHN model. The predicted leachate volume and the NO3-N concentration in the leachate are then used to predict the likely impact on groundwater contamination. If the NO3-N concentration in the leachate is below the WHO maximum permissible limit (MPL) of 11.3 mg/L in drinking water, then the

Check the long-term effects of the current irrigation plan

Run LEACHN model Monthly weather data, soil

temperature, soil moisture and nutrient contents of the

starting day, potential ET (weekly total)

Irrigation volumes at planned frequency

Predicts: Monthly total ET, leachate, soil moisture and nutrient contents, and plant uptake of nutrients

Is significant pollution of groundwater likely to occur?

Yes No

Modify the irrigation scenario and re-run the

model Continue planned irrigation Output Input Input

planned irrigation scenario can be implemented. If the model suggests that the leachate NO3-N concentration is likely to exceed considerably the MPL (i.e. there is a risk of unacceptable groundwater contamination), then the planned irrigation scenario can be modified and the model re-run.

Prior to applying the LEACHN model to the data from the field trial described in Chapter 3 a short term, laboratory-based lysimeter study was used to test the ability of the model to predict the leaching of NO3-N through soil.

5.3 Methodology

Two “undisturbed” soil lysimeters were collected from a grazed pasture at Massey University. Cameron et al. (1992) have given detailed descriptions of lysimeter collection and installation. The soil was Manawatu fine sandy loam. Particle size distribution data for this soil type were obtained from the National Soils Database (NSD-Landcare Research Institute, New Zealand).

Two independent cylindrical lysimeters (A and B) of 400 mm diameter and 600 mm depth were created by gradually pushing the lysimeter casing down into the ground. The lysimeter casings were made of PVC pipe. The edge of the lysimeter casing was bevelled to a 450 angle using an angle grinder. The bottom 3 - 5 cm of soil in the lysimeter was replaced with gravel to hold the soil and collect the filtered leachate. The gap between the soil core and lysimeter casing was sealed using a snow-white petrolatum to prevent edge flow effects (Cameron et al., 1992).

The area of each lysimeter was 0.1256 m2. The lysimeters were set up on 18th August 1999 in a glasshouse at the Plant Growth Unit at Massey University. Each lysimeter had pasture growing on the soil surface. Prior to the first irrigation, the pasture was cut to a height of 10 mm in each lysimeter and it was then cut again to 10 mm after 10 weeks. The temperature of the air surrounding the lysimeters varied within the range 18 - 25 0C (Peter Kemp, personal communication, 1999).

Lysimeters A and B were used to calibrate and validate the model, respectively. The calibration period, using lysimeter A, lasted 72 days (from 19th August to 29th October, 1999). Prior to the first irrigation, measurements of SMC, soil nitrogen and carbon contents, bulk density, and soil temperature were made in the top 500 mm of the soil profile for lysimeters A and B. The detail of how the soil samples were collected is given later.

Treated dairy shed effluent (DSE) (collected from the No. 4 dairy farm at Massey University) was applied at the designed rate of 3.6 mm/week on lysimeter A. The depth of effluent irrigation (3.6 mm/week) was calculated on the basis of the estimated average inorganic N concentration of the DSE (0.08 kg/m3) and the desired N loading rate of 150 kg/ha/year - as per Wellington Regional Council Rule 11 (Annual Wellington Regional Council Plans, 1997).

DSE was applied for 5 - 10 minutes using a hand spray bottle in order to approximately simulate spray irrigation. DSE samples were collected on a weekly basis from the oxidation pond. All the effluent samples were stored at 4 0C until analysed for NO3-N and NH4-N using the standard methods of water analysis (Gillian, 1984). Artificial “rainfall” was applied to the top of lysimeter A, 24 hours after each DSE irrigation event (using the same hand spray bottle). Then sufficient rainfall was added to generate leachate. The amount needed was based on measurements of SMC and PET, and varied between 16 and 56 mm/week.

TDR probes were installed vertically permanently at five depths (0 - 50, 0 - 150, 0 - 250, 0 - 350, and 0 - 450 mm) in lysimeter A. Volumetric SMC measurements were made at these depths prior to irrigation to provide the initial SMC input values to LEACHN. SMC measurements were also made after the rapid drainage of soil water to estimate the FC of the soil. Soil temperature measurements were also made at the same depths prior to the first irrigation and then weekly in the top 150 mm of the soil profile using a temperature probe permanently installed in lysimeter A.

Soil core samples were collected at the same depths (i.e. 50, 150, 250, 350, and 450 mm) from the pasture adjacent to where the lysimeter samples (A and B) were taken, in order to provide initial soil nutrient values for the model. The samples were analysed for NO3-N, NH4-N, and organic carbon using the standard methods of Blakemore et al. (1987). The bulk density of the soil samples was determined using the standard method of Blakemore et al. (1987).

A pan evaporator was placed in the glasshouse to measure the pan evaporation rate. A crop factor of 0.75 was used to calculate the PET on a weekly basis for the pasture crop. Leachate samples were collected after each “rainfall” event. All the leachate samples were stored at 40 C until analysed for NO3-N and NH4-N using the standard methods of water analysis (Gillian, 1984).