Data to Support the Concept

This concept was tested using data from the field study at the Carterton land treatment site.

4.2.3.1 Water Balance Approach to Estimate Drainage Losses

The components of the water balance that were monitored at the site were: rainfall

volume of effluent irrigation applied

stored soil moisture (through soil moisture content measurements) pan evaporation data (collected from a weather station close to the site)

The following equation was used to estimate the drainage losses between times t1 and t2. This water balance was calculated biweekly.

Where,

SSMt1 = Stored Soil moisture on day t1 (mm) SSMt2 = Stored Soil moisture on day t2 (mm)

IRR (net) = Net irrigation applied during time t1 to t2 (mm) PPT (net) = Total rainfall that occurred during time t1 to t2 (mm)

AET (net) = Estimated actual water use by trees or pasture during time t1 to t2 (mm)

Drainage losses from the tree plantation area: The drainage loss estimations for the Eucalyptus tree plantation area were made using Equation 4.1. The results (Fig. 4.2) indicate that there would be a marked irrigation treatment effect on the estimated drainage losses i.e. the volume of drainage water (mm) increased with the increase of irrigation volume. There were no predicted drainage losses in the L and M treatments during summer and early autumn (December - March) - except for a brief event in early December when there was 67 mm of rain (Fig. 4.2). From late autumn to early spring (April - August), there were predicted drainage losses in the L and M treatments because of high rainfall, and soil moisture contents that were close to FC. There were predicted drainage losses in the H treatment throughout the irrigation period. These were attributed to the rainfall events plus the high effluent application rate. The small negative drainage values over summer in the L and M treatments (Fig. 4.2), result from over-estimation of AET from dry soils, and can be considered to represent nil drainage.

Fig. 4.2: Rainfall and estimated drainage losses (mm/2-weeks) in the low, medium and high effluent irrigation treatments on the tree plantation area.

It is important to note that this water balance approach of estimating drainage losses may result in an under-estimate, because there might be some drainage occurring from preferential flow of effluent during rain-free periods. Nevertheless, it provides a useful first approximation and was used to estimate the drainage losses in all treatment areas. For this particular year, the water balance suggested that up to 45 mm of effluent could be applied to the tree plantation area per week during summer and early autumn without causing drainage that could carry NO3-N beyond the reach of plant roots. During late autumn and winter leaching was predicted to occur – even in the L treatment.

Drainage losses from the pasture area: The estimated drainage losses from all treatments on the pasture area are shown in Fig. 4.3. Once again there was a predicted irrigation treatment effect on the drainage losses but in all cases the predicted leaching losses from the pasture plots (Fig. 4.3) were higher than from the tree plots (Fig. 4.2). During summer there were only small drainage losses from the L irrigation treatment, and during this time effluent could be applied at a rate of 30 mm/week to pasture without having a risk of major drainage losses, depending again on the soil hydrological and climatic conditions. During autumn and winter, when SMC was high and there were frequent rainfall events, drainage is very likely to occur irrespective of the rate of effluent application.

Fig. 4.3: Rainfall and estimated drainage losses (mm/2-weeks) in the low, medium and high effluent irrigation treatments on the pasture area.

High

SMC and NO3-N concentration in groundwater - Figure 4.4 shows the pattern of SMC and groundwater NO3-N concentration in the L, M and H effluent irrigation treatments on the tree plantation areas. During summer and autumn, when there was little rain and the SMC was low (10 - 25%) in all treatments of the tree plantation area, the NO3-N concentration in groundwater was below the MPL (i.e. 11.3 mg/L). During this time, the fortnightly drainage losses were also low (0 - 40 mm, 0 - 77 mm and 50 - 205 mm in the L, M and H effluent irrigation treatments, respectively – see Figure 4.3).

During winter, when rainfall was higher and the soil moisture content was between 32 and 35% (i.e. close to FC) in all treatments of the tree plantation area, the NO3-N concentration in the shallow groundwater immediately beneath the plots exceeded the MPL. This increase in NO3-N concentration was attributed to leaching occurring in response to rainfall when the SMC was high.

Fig. 4.4: Soil moisture content, NO3-N concentration in groundwater and the maximum permissible limit for NO3-N concentration in groundwater for the tree plantation receiving different rates of effluent irrigation.

This exercise helped in understanding the key mechanism governing the distribution and transportation of N through the soil-water matrix into the groundwater at this site. In summary, monitoring of SMC along with predicted rainfall would enable

Medium

High Low

the manager of a LTS to predict early the NO3-N leakage at the site and make short term management decisions to minimize this.

4.2.4 An Assessment of the Importance and Practical Feasibility of