Predicting the effects of landuse on water quality – Stage I
Enterprise-scale modelling for horticulture (HortResearch, Objective 4b)
The Soil Plant Atmosphere System Model (SPASMO) can be used to estimate N leaching from land being used for horticultural cropping. In this first year of the project, it is being used to generate information on N leaching for case studies, which have then been used for the triple bottom line modelling work (Section 10). The SPASMO model is the most numerically detailed of all the models in this project, and requires a long sequence of site-specific daily climate information as input data. Links between the SPASMO model and the catchment modelling framework (Section 5) are not yet clearly defined. Ideally, the SPASMO model would be linked into the modelling framework in the same way as OVERSEER, that is, as a model which can be run automatically from the framework, for representative case studies that cover all horticultural crops in the catchment of interest.
In the initial phase of this work, carried out under contract to NIWA, we have run our Soil Plant Atmosphere System Model (SPASMO Version W1.2) for five enterprise scenarios (each with some internal variations) over 32 years. The output data from these simulations is presented in the form of a daily time-series of the nitrogen concentration in the soil solution, in mg-N/L, as it leaches below the root zone. Also provided is a statistical analysis of the cumulative probability of exceedence of the N-concentration in that leachate water which enters groundwater, at the specified depth. Because the flux of drainage water is also calculated as part of the SPASMO simulations, the calculations can also be given in terms of the loading of nitrogen (kg/ha/day) upon the groundwater. Details of the SPASMO model are given in Section 13 of this report. The five scenarios we have examined are:
Grapes - Marlborough (Fairhall stony silt loam) with a water table at 2, 3, and 4 m deep, and 20 kg-N/ha of Calcium Ammonium Nitrate fertilizer (Ca.NH4.(NO3)2) applied in mid-October. The varying water table reflects local variations to cover the range experienced in this local region.
Grapes - Hawkes Bay (Maraekakaho) with a water table at 3, 5 and 7 m, and 30 kg - N/ha of CAN applied in mid-October. This covers water-table variations experienced in this area.
Kiwifruit - Bay of Plenty (Maketu) with a shallow water table at 3.5 m and totals of 100, 200 or 400 kg - N/ha applied in two split applications: September and November.
Apples - Hawkes Bay (Twyford) with a water table at 3 m and 50 or 100 kg - N/ha applied in spring.
Potatoes - Waikato (Matamata) with 200 kg - N/ha applied over the year according to grower practice with a water table at 3 m.
Results are given below for each of the five scenarios.
Grapes - Marlborough
Grapes - Marlborough (Fairhall stony silt loam) with a water table at 2, 3, and 4 m deep, and 20 kg-N/ha of calcium ammonium nitrate (CAN) applied in mid-October.
The results from applying our SPASMO model to these scenarios are shown in Figure 8-1 and Figure 8-2 for nitrate-N, and summarised in Table 8-1. As part of these calculations, ammonium-N concentrations are always calculated, although these are not presented here. In Figure 8-1, the daily nitrate-N concentrations are shown for a 10-year window.
Figure 8-1: The SPASMO-Predicted concentration of nitrate at a depth of 2.0, 3.0 and 4.0 m under a vineyard growing on the Fairhall stony silt loam in Marlborough. A single dose of CaNH4(NO3)2 fertilizer was applied at a rate of 20 kg-N/ha on the 15th October each year. The World Health Organization has set the maximum allowable value (MAV) of 11.3 mg-N/L for nitrate in the drinking water. While the nitrate levels are elevated, the concentrations are less than half the MAV.
Figure 8-2: Probability of exceedence for nitrate concentration under a vineyard growing on the Renwick stony silt loam in Marlborough. The nitrate-N concentration below the root zone is almost always less than the MAV of 11.3 mg-N/L. The concentration does not change much with depth because denitrification losses are small below the root zone. Nitrate poses a small threat of contamination to the shallow groundwater.
Table 8-1: Annual nitrogen budget for a grape vineyard in Marlborough. Total nitrogen uptake by the grape vines is 54 kg/ha. Some 13 kg is in the harvested fruit, 50 kg N/ha is returned to the soil in the form of leaves and roots, and winter prunings, and ~13 kg/ha is lost back to the atmosphere as volatilisation and denitrification. In this scenario, surplus nitrogen is consigned to drainage water.
| Nitrogen budget of grapes [kg/ha N] | |
|---|---|
| Fertilizer | 20 |
| N uptake | 54 |
| fruit | -13 |
| recycled | 50 |
| volatilization | -7 |
| denitrification | -6 |
| mineralization | 44 |
| drainage | -10 |
In Figure 8-2, the cumulative distribution of the probability of exceedence is shown for nitrate-N, considering the water-table to be at various depths. These data derive from the nearly 12,000 daily-values simulated over the 32-year period using actual weather data from Woodbourne
Grapes - Hawkes Bay
Grapes - Hawkes Bay (Takapau sandy loam) with a water table at 2, 3 and 4 m, and 20 kg - N/ha of CAN applied in mid-October. This scenario covers the shallow water-table variations experienced in this area.
A SPASMO simulation was carried out to consider the establishment of a new vineyard at Maraekakaho (Figure 8-3). New vineyards are being developed in this area, where the grapes are grown on permeable soils overlying an unconfined aquifer.
Figure 8-3: Predicted concentration of nitrate in the soil under a vineyard at Maraekakaho that receives an annual dressing of nitrogen fertilizer at a rate of 20 kg-N/ha. The concentration is trending towards the WHO drinking water standard of 11.3 mg-l for nitrate.
As can be seen from Figure 8-3, as the vineyard becomes established, the fertiliser practices of just 20 kg-N/ha/yr will lift leachate levels close to the WHO limit, depending on the depth of the aquifer. Only about 15 kg-N/ha is removed in the grape berries, and all the prunings are returned to the soil. These results are summarised in Table 8-2
Table 8-2: Annual nitrogen budget for a grape vineyard near Maraekakaho. Total nitrogen uptake by the grape vines is 75 kg/ha. Some 15 kg is in the harvested fruit, 61 kg N/ha is returned to the soil in the form of leaves and roots, and winter prunings, and ~17 kg/ha is lost back to the atmosphere as volatilisation and denitrification. The leaching losses under a vineyard at Maraekakaho is calculated to be some 17 kg N/ha, on average, each year.
| Nitrogen budget of grapes [kg/ha N] | |
|---|---|
| Fertilizer | 20 |
| N uptake | 75 |
| fruit | -15 |
| recycled | 61 |
| volatilization | -11 |
| denitrification | -6 |
| mineralization | 77 |
| drainage | -17 |
Kiwifruit - Bay of Plenty
Kiwifruit - Bay of Plenty (Maketu) on a Katikati silt loam with totals of 200 and 400 kgN/ha as CAN applied in two split applications: September and November. Vines were irrigated using 25 mm of water applied on the basis of need. Elevated levels of nitrate exceeding the MAV value of 11.3 mg/L nitrate-N are predicted more than half of the time (Figure 8-4, Figure 8.5 and Figure 8.6). There is a 10% probability that the soil-nitrate solution concentration will exceed 20 mg/L. The results are summarised in Table 8-3
Figure 8-4: Predicted concentration of soil nitrate in drainage water under a kiwifruit vineyard at Maketu near Te Puke. An annual dressing of nitrogen fertilizer has been applied at a rate of 200 kg-N/ha.
Figure 8-5: Probability of exceedence for nitrate concentration under a kiwifruit vineyard at Maketu near Te Puke that receives 200 kgN/ha nitrogen fertilizer. The nitrate-N concentration below the root zone exceeds the MAV of 13.2 mg-N/L more than half of the time. There is a 10% probability that nitrate concentration exceeds 20 mg/L. Nitrate poses some threat of contamination to the shallow groundwater.
Figure 8-6: Probability of exceedence for nitrate concentration under a kiwifruit vineyard at Maketu near Te Puke that receives an annual dressing of 400 kg-N/ha as CAN. The nitrate-N concentration below the root zone exceeds 39 mg/L more than half of the time. Nitrate poses a much greater threat of contamination to the shallow groundwater.
Table 8-3: Annual nitrogen budget for a kiwifruit vineyard at Maketu near Te Puke. Total nitrogen uptake by the vines is 165 kg/ha. Some 68 kg is in the harvested fruit, 96 kg N/ha is returned to the soil in the form of leaves and winter prunings, and 60-80 kg/ha is lost back to the atmosphere as volatilisation and denitrification. In this scenario, extra nitrogen fertilizer is consigned to drainage.
| Nitrogen budget of kiwifruit [kg/ha/y N] | ||
|---|---|---|
| Fertilizer | 200 | 400 |
| N uptake | 165 | 164 |
| Fruit | -68 | -68 |
| Recycled | 96 | 96 |
| Volatilization | -24 | -44 |
| Denitrification | -33 | -45 |
| Mineralization | 103 | 103 |
| Drainage | -106 | -295 |
Apples - Hawkes Bay
Apples - Hawkes Bay (Twyford) with a water table at 3 m and a nitrogen fertilizer application of 50 & 100 kg - N/ha applied every year in spring. The soil is a Takapau sandy loam (80 % stones beyond 1 m depth). Irrigation of 25 mm is applied each time, on the basis of need. Apple yield is about 50 T/ha. The annual nitrogen balance is presented in Table 8-4. The nitrogen concentration in the drainage water quitting the root zone is low and always less than the MAV of 11.3 mg/L under the 50 kg/ha fertilizer scenario. Figure 8-7, Figure 8-8, Figure 8-9, and Figure 8-10 give details.
Table 8-4: The annual nitrogen budget for an apple orchard near Hastings. Total nitrogen uptake by the trees is calculated to be 185-200 kg/ha. Some 60 kg of nitrogen is in the harvested fruit, 133 kg N/ha is returned to the soil in the form of leaves and winter prunings, and 18-26 kg/ha is lost back to the atmosphere as volatilisation and denitrification. Nitrate leaching losses are low (~7 kg-N/ha) for the low fertilizer input, and are moderate (35 kg-N/ha) for the high fertiliser input.
| Nitrogen budget of apple [kg/ha/y N] | ||
|---|---|---|
| Fertilizer | 50 | 100 |
| N uptake | 185 | 200 |
| Fruit | -60 | -60 |
| Recycled | 133 | 133 |
| Volatilization | -15 | -18 |
| Denitrification | -3 | -8 |
| Mineralization | 159 | 159 |
| Drainage | -7 | -35 |
Figure 8-7: Predicted concentration of soil nitrate in drainage water under an apple orchard near Hastings. An annual dressing of nitrogen fertilizer has been applied at a rate of 50 kg-N/ha. The soil is a free-draining Takapau silt loam with coarse gravel (> 80% stones) beyond a depth of 1.0 m.
Figure 8-8: Probability distribution for nitrate concentration under an apple orchard near Hastings that receives an annual nitrogen application of 50 kg-N/ha. The nitrate-N concentration below the root zone exceeds 2.8 mg/L more than half of the time. Nitrate poses little threat of contamination to the shallow groundwater since the concentration in the drainage water is always less than the MAV of 11.3 mg/L.
Figure 8-9: Predicted concentration of soil nitrate in drainage water under an apple orchard near Hastings. An annual dressing of nitrogen fertilizer has been applied at a rate of 100 kg-N/ha. Nitrate poses a risk of contamination to the shallow ground water.
Figure 8-10: Probability distribution for nitrate concentration under an apple orchard receiving 100 kg-N/ha nitrogen fertiliser. The nitrate-N concentration exceeds 15 mg/L some 50% of the time. Nitrate in the drainage water could a threat of contamination to the shallow groundwater. However, drainage losses (see Table 8-4) are moderate because of the low rainfall in the Hawkes Bay.
Potatoes - Waikato
Potatoes - Waikato (Matamata) with 200 kg - N/ha applied over the year according to grower practice with a water table at 3 m. The soil is a free-draining Horotiu silt loam. Potatoes have a tuber yield of 50T/ha, a dry matter content of 20% and a nitrogen content of about 1.8% in the tubers. See Figure 8-11 for details, and Table 8-5 for a summary of the results.
Figure 8-11: Predicted concentration of soil nitrate in drainage water under a potato field near Matamata. The soil is a free-draining Horotiu silt loam. An annual dressing of nitrogen fertilizer has been applied at a rate of 200 kg-N/ha. Nitrate poses a risk of contamination to the shallow ground water.
Table 8-5: The annual nitrogen budget for a potato crop near Matamata. Total nitrogen uptake by the crop is calculated to be 136 kg-N/ha. Some 85 kg of nitrogen is in the harvested in the tubers, 51 kg N/ha is returned to the soil in the form of leaves and winter prunings, and some 59 kg/ha is lost back to the atmosphere as volatilisation and denitrification. Nitrate leaching losses are high (> 100 kg/ha/year) for this level of fertilizer input. A cover crop would normally be used to mop up this excess nitrogen. Here we have simulated the worst case scenario, with bare soil between annual crops.
| Nitrogen budget of potato [kg/ha/y N] | |
|---|---|
| Fertilizer | 200 |
| N uptake | 136 |
| Tubers | -85 |
| recycled | 51 |
| volatilization | -36 |
| denitrification | -23 |
| mineralization | 94 |
| drainage | -113 |
Summary of Results for Horticulture Landuse
The average annual leaching losses of nitrate (kg N/ha) are presented in Table 8-6 for a range of horticultural crops. In the case of grapes receiving an annual dressing of some 20 kg N/ha, the annual leaching losses are quite small (i.e. 10-17 kg N/ha). This is because the additional nitrogen fertilizer matches approximately the amount of nitrogen taken off in the grape crop. Similarly, low levels of nitrate leaching (~ 7 kg N/ha) are expected under apples in the Hawkes Bay when nitrogen fertilizer is applied at a rate of just 50 kg N/ha. However, if nitrogen fertilizer is applied at a rate that exceeds the annual crop requirement, than the excess is available to leach. Three scenarios in Table 8-6 (i.e. apples at 100 kg N/ha and kiwifruit at 200 & 400 kg N/ha) have annual leaching losses that will impact on the quality of the shallow ground water. The modelling and risk assessment tools being developed here will help identify those at risk locations and/or poor fertilizer practices.
Table 8-6: Summary of the average annual nitrogen budget for horticultural land use.
| Crop | Location | Soil series | Fertilizer [kg N/ha] | Uptake [kg N/ha] | Crop [kg N/ha] | Drainage [kg N/ha] |
|---|---|---|---|---|---|---|
| Grapes | Marlborough | Renwick | 20 | 54 | 13 | 10 |
| Hawkes Bay | Takapau | 20 | 75 | 15 | 17 | |
| Kiwifruit | Bay of Plenty | Katikati | 200 | 164 | 68 | 106 |
| 400 | 165 | 69 | 295 | |||
| Apples | Hawkes Bay | Twyford | 50 | 185 | 60 | 7 |
| 100 | 200 | 60 | 35 | |||
| Potatoes | Waikato | Horotiu | 200 | 136 | 85 | 113 |
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