OVERSEER® NUTRIENT BUDGETS AND
IRRIGATION
David Wheeler1, John Bright2
1AgResearch, Hamilton
2Aqualinc, 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:
SMt = SMt-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
Saturation
Irrigation
Field capacity
AWC
Wilting point
0.95
Target
0.70
Trigger
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
Important to align climate and
irrigation rates
E x a m p le : W a ik a to
20
18
P ro p o rtio n o f a n n u a l ra in fa ll (% )
Overseer climate database uses
a ‘typical’ distribution of
rainfall, PET and temperature:
• Currently typical long-term
patterns
• Moving to site-specific
long-term patterns
L o n g -te rm a ve ra g e
E xa m p le ye a r 2 0 0 8
16
14
12
10
8
6
4
2
0
ja n
fe b m a r a p r m a y ju n
ju l
a u g se p
o ct
nov dec
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
800 mm rainfall, irrigated October to February central pivot
Low AWC
(56 mm)
High AWC
(104 mm)
Irr *
Drain**
Irr *
Drain **
Default
258
247
230
205
20 mm depth, 4 day return
744
729
764
739
15 mm depth, 4 day return
558
543
573
548
20 mm depth, 11 day return
261
265
261
237
50 mm depth, 11 day return
653
575
704
567
Variable (50 - 95% AWC)
255
243
224
190
Add at 50% AWC, 50 mm depth
402
388
251
225
Add at 50% AWC, 100 mm depth
804
403
503
225
* Estimated irrigation rate (mm/yr)
** Estimated drainage (mm/yr)
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