OVERSEER® NUTRIENT BUDGETS AND IRRIGATION David Wheeler 1, John Bright 2 1 AgResearch, Hamilton 2 Aqualinc, Christchurch

BACKGROUND Overseer is a nutrient management tool Drainage is an important driver of leaching Irrigation inputs accounted for in the drainage calculation Concerns about use of irrigation in Overseer Aligning irrigation inputs to climate data inputs Type of management options within Overseer

DRAINAGE MODEL Each day: SM t = SM t-1 + rainfall + irrigation – AET – drainage – runoff Drainage occurs when soils exceed field capacity The amount and timing of irrigation can affect drainage Climate data determines rainfall pattern, potential evapotranspiration

INPUTTING IRRIGATION DATA INTO OVS ‘Method only’ ‘Method plus rate’

USING ‘METHOD ONLY’ Overseer automatically calculates the irrigation amount required to maintain high soil moisture content while minimising drainage Field capacity Wilting point Saturation AWC Trigger Target Irrigation

USING ‘METHOD ONLY’ Includes an estimate of ‘system losses’ –Losses due to leaks, overlaps, inefficiencies, etc –Added to drainage –Border dyke also includes outwash loss Active management check box –Calculation then based on: » No system losses assumed » No irrigation applied within 5 days of rainfall Requires nutrient concentrations in water –Border dyke outwash can be recycled

USING ‘METHOD PLUS RATE’ A three step process: 1.Irrigation amount estimated as per ‘method only’ 2.Applied rate estimated from user-entered rate data 3.If estimated applied rate > method only rate Difference added to drainage ESSENTIAL that entered rate and climate data align

CLIMATE DATA User inputs to describe climate are: Annual rainfall Annual PET Annual average temperature Default PET and temperature based on long-term climate data set User can define period annual data is average of, but Data must be commensurate with the objectives of setting up the Overseer file, and with management input data

CLIMATE PATTERNS Overseer climate database uses a ‘typical’ distribution of rainfall, PET and temperature: Currently typical long-term patterns Moving to site-specific long-term patterns Important to align climate and irrigation rates Illustration of long-term average rainfall patterns and variation with a single year

NEW WORK UNDERWAY The following projects will add robustness to the use of irrigation in Overseer: Comparison of drainage estimates under irrigation between Overseer and IrriCalc Also looking into additional management options to describe irrigation use Improving supporting databases: Soil data Default climate data

IRRIGATION MANAGEMENT Test bed

EFFECT OF MANAGEMENT RULES ON CALCULATED DRAINAGE Low AWC (56 mm) High AWC (104 mm) Irr *Drain**Irr *Drain ** Default mm depth, 4 day return mm depth, 4 day return mm depth, 11 day return mm depth, 11 day return Variable ( % AWC) Add at 50% AWC, 50 mm depth Add at 50% AWC, 100 mm depth * Estimated irrigation rate (mm/yr) ** Estimated drainage (mm/yr) 800 mm rainfall, irrigated October to February central pivot

CONCLUSIONS Recommend use ‘method only’ for now This leaves Overseer to estimate the irrigation rate Minimises the risk of not aligning irrigation rate and rainfall If using ‘method plus rate’, then align climate and irrigation rate Not recommended Using this option may lead to an overestimation of drainage Irrigation management rules are important Investigating how to increase range of rules in Overseer Look out for further updates As we work through the results of the Irricalc comparison project

ACKNOWLEDGMENTS OVERSEER® owners are Ministry for Primary Industries, Fertiliser Association of New Zealand and AgResearch