Measuring & modelling soil water balance and nitrate leaching of perennial crops in New Zealand

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1 Measuring & modelling soil water balance and nitrate leaching of perennial crops in New Zealand Steve Green, Brent Clothier, Karin Müller

2 Key facts: Water allocation in New Zealand Abundant freshwater but not always where & when it is needed Increasing local groundwater depletion & reduced recharge 721 kha irrigable land in 2012 Potential increase: 1.5 Mill ha (dairying, viticulture & crops) Irrigation accounts for 51% of allocated consumptive water use (mainly groundwater) Consumptive water demand 2-3 x higher than OECD average Canterbury: 64% of total water allocated for irrigation Many other catchments (e.g., Hawke s Bay, Wairarapa, Marlborough, Otago) are heavily or fully allocated Sources: MPI: Environment Aotearoa 2015; MPI: Our fresh water 2017; NZ Statistics 2012, Agricultural Production Census; OECD Compendium of Agri-environmental Indicators 2013

3 Key facts: Water quality in New Zealand Water quality in lakes, rivers, streams & aquifers is variable Mainly dependent on dominant land use in catchment Major pressure on fresh water: agricultural intensification Nitrogen leaching increased by 29% from 1990 to 2012 River sites monitored for NO 3 -N: 55% worsening, 28% improving DRP: 25% worsening, 42% improving 32% of monitored river sites enough DRP to trigger nuisance periphyton growth Sources: MPI: Environment Aotearoa 2015; MPI: Our fresh water 2017

4 Nitrate levels in groundwater & trends in rivers

5 Resource Management Act (1991) - NZ s town and country planning law Purpose: to promote the sustainable management of natural and physical resources Water allocation: No one can take, use, dam or divert water unless allowed by: A National Environmental Standard A rule in a plan A resource consent (water permit) Water quality: Regional Councils make Regional Plans for safeguarding the life-supporting capacity of air, water, soil and ecosystems Local solutions for local problems: diversity of approaches to avoid/remedy impacts of nutrient leaching from farms on WQ

6 National Policy Statement for Freshwater Management National Bottom Lines for Water Quality - A National Objectives Framework Regional Councils to develop regulations by 2025 that ensure these are met - no inclusion of GW-quality - no P attribute

7 Case study: Nitrate leaching under vineyards, Wairau Plains, Marlborough, South Island Objectives: Quantify influence of nutrient losses on river quality Calibrate model against ground truth data Model influence of potential land use change on water quality

8 Modelling and measuring approach to evaluate nutrient losses draining from a vineyard to the water table Wairau aquifer is fast flowing with almost direct connection to the Spring Creek

9 Drainage flux meters to measure drainage Convergence ring 200 mm 150 mm The drainage flux of water (D W, mm/d) over the time interval (Δt, day), is calculated as: Wick material 50 mm Air inlet tube 1000 mm The leaching flux of nutrients (L N, kg/ha/d) is calculated as: Tipping spoon Water outlet tube 150 mm

10 TDR sensors to measure soil water contents Rainfall Soil water content [V/V] Rainfall [mm/day] 0.00 May Aug Nov Feb May Aug Nov Feb May Aug Nov Feb May Aug Rainfall Time Domain Reflectometry (TDR) sensors installed from soil surface at different depths (to 1.8 m) Changes in soil water content measured daily to track the soil water balance Soil water content [V/V] May Aug Nov The Feb New Zealand May Aug Institute Nov for Plant Feb & Food May Research Aug Limited Nov Feb May Aug Rainfall [mm/day]

11 Sap flow sensors to measure vine water use Heat-pulse technique to monitor sap flow in plant Summed up to provide measure of daily transpiration (dots) Vine water use is modelled from potential ET and vine leaf area using a crop factor approach (line)

12 Modelling nutrient fate under perennial crops Soil Plant Atmosphere System Model (SPASMO) crop (pasture, tree, field crop) FAO-56 approach Bare soil Grass inter-row Fine sandy loam Fine sand 3 Overland flow: SCS Curve Number Approach (Williams, 1991) Coarse gravel Soil water balance: Daily times steps Vertical water flow: Dual-porosity model (Hutson and Wagenet, 1993)

13 Input parameter: Properties of soil horizons Measurements & digital soil maps Depths of soil horizons Soil texture Stone content Organic carbon content Bulk density Define soil hydraulic properties Water content [L/L] saturation Pressure head [cm] Field capacity Refill point Wilting point sandy loam coarse sand Air dry

14 Input parameter: Climate data Radiation, wind speed, temperature, humidity, rainfall Weather stations: measurements Virtual Climate Database (NIWA)

15 Input parameter: Crop growth & management leaf DM shoot DM bunch DM Trimming (LAI ~ 1.0) Leaf fall plant DM [kg/ha] Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Winter pruning phenology GDD growth f(rg, Ta, W, N) uptake f(supply,demand) budburst flowering harvest Bub break [%] Flowering [%] Brix [%] GDD sum (degree-day) GDD sum (degree-day) GDD sum (degree-day)

16 Soil water balance of the vineyard SWC model SWC data DR Model DR data Soil water content [mm] Drainage [mm] 200 mm 150 mm 150 mm 50 mm 1000 mm 0 Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul 0 Vine water use has been modelled using a dual crop factor changes in soil moisture and drainage rates match observations

17 Seasonal pattern of nitrate concentrations selected DFMs all DFMs 6.0 nitrate-n [mg/l] Apr12 Jul12 Oct12 Dec12 Apr13 Jul13 Oct13 Jan14 Apr14 DFM devices are capturing water that drains from vineyard Elevated levels of nitrate are seen at a depth of 1.2 m

18 Probability of exceedance for nitrate concentrations modelled using 30 years of climate data Not much change with depth (denitrification losses small below roots) Concentrations almost always below NZ drinking water standard Modelled loss: 16 kg N/ha yr

19 Leaching of nitrate is occurring from vineyards Wratts Road DFM-data 1.2 m depth 200 mm Nitrate-N (mg/l) Groundwater 2.4 km away 0.0 Oct06 Oct07 Oct08 Oct09 Oct10 Oct11 Oct12 Oct mm Levels of nitrate in drainage water are greater than in groundwater Use DFM data to explore the linkage (attenuation and time delay)

20 Attenuation and delay of nitrate to aquifer Wratts Road Model Nitrate-N (mg/l) Oct06 Oct07 Oct08 Oct09 Oct10 Oct11 Oct12 Oct13 ANZECC Surface Water Guideline for ecosystem health A simple time delay and attenuation model was used to estimate the groundwater concentrations as: The groundwater lags leaching by 185 days. And the good news... the GW concentration is just 10% of that leaching.

21 Summary of case study Trials have been successful at determining nitrate losses Provide a baseline for nitrate losses from viticulture which covers most of the Plains Monitoring will continue to capture more diverse range of rainfall Council has long-term (>20 yr) records from some wells changes in groundwater quality over time and some hot spots for nitrate Can explain changes in groundwater quality using a simple approach Next stage is to combine our model with a dynamic geohydrological model to simulate interactions between land use and groundwater quality

22 Acknowledgement the support of Marlborough District Council Rachel Rait & Peter Davidson Further contact: Dr Steve Green, Plant & Food Research, Tennent Drive, Palmerston North, NZ