§ 17

Watering account: water balance for a period

Should the fairway be watered after a week with 21 mm evaporation and 6 mm rain?

Answer: A watering account keeps track of how much water the root zone lacks. Grass uses water according to the potential evaporation which DMI publishes daily, times a crop coefficient. Rain and watering fill up again. When the shortfall reaches the limit you allow the root zone to dry to, you must water. The calculator works out one period, eg a week, and tells you whether and how much to water. It doesn't replace a soil moisture metre but it gives you the figure for the watering plan.

Formula

Græssets fordampning = Kc × potentiel fordampning (DMI udgiver den potentielle fordampning pr. dag for hele landet)

Underskud efter perioden = underskud ved start + græssets fordampning − nedbør − vanding (aldrig under 0 og aldrig over rodzonens vand)

Vandingsgrænse = plantetilgængeligt vand i rodzonen × tilladt udtørring

If the shortfall has reached the limit, the shortfall is topped up again. Otherwise you wait

FAO-56 table 12: cool season grass Kc 0.90 early and 0.95 mid season (grass cut at 6-8 cm). Adjust based on your own soil moisture measurements

Symbols and units in the formula
Symbol	Meaning	Unit
start_mm	Shortfall at start of period	mm
ep_mm	Potential evaporation in the period (DMI)	mm
kc	Crop coefficient for grass (Kc)
nedboer_mm	Rainfall in the period	mm
vanding_mm	Watering you have already given in the period	mm
rodzone_mm	Plant available water in root zone	mm
tilladt_pct	Permitted drying before watering	%
=	Shortfall after period	mm
=	Water you should give now	mm
=	Graessets fordampning i perioden	mm
=	Watering limit (permitted deficit)	mm
=	Space for more drying before limit	mm

Validity and source

Source: Water balance model with DMI's potential evaporation and FAO-56 crop coefficient (cool-season grass Kc 0.95)

Does not apply when

Rain falling as downpour partly runs off and doesn't count fully. Plant available water in root zone depends on soil type and root depth: a sand green with 10–15 cm roots holds far less than a fairway on humus soil. Check with the soil moisture meter.

Worked example

The calculator's own default values inserted. The figures are calculated with exactly the formula the tool uses.

Substituted

Shortfall at start of period5 mm
Potential evaporation in the period (DMI)21 mm
Crop coefficient for grass (Kc)0.95
Rainfall in the period6 mm
Watering you have already given in the period0 mm
Plant available water in root zone30 mm
Permitted drying before watering50 %

Giver

Shortfall after period19.0 mm
Water you should give now19.0 mm
Graessets fordampning i perioden20.0 mm
Watering limit (permitted deficit)15.0 mm
Space for more drying before limit0.0 mm

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Sådan bruger du den

Et vandingsregnskab sparer vand og holder græsset sundt. Du lægger græssets fordampning til underskuddet og trækker regn og vanding fra. Når underskuddet når den grænse, rodzonen må tørre ud til, skal der vandes. Det bruges hver uge eller dag i sæsonen.

Step by step

Find den potentielle fordampning for perioden hos DMI.
Gang med afgrødekoefficienten for græs (FAO-56: 0,95 midt i sæsonen).
Læg til underskuddet ved periodens start og træk nedbør og vanding fra.
Find vandingsgrænsen: rodzonens plantetilgængelige vand × tilladt udtørring.
Er underskuddet over grænsen, så vand underskuddet op igen.

Typical errors

At tælle al regn med. Kraftig regn løber delvist af og fylder ikke rodzonen.
At bruge samme rodzone for greens og fairways. En sandgreen rummer langt mindre vand.
At vande lidt hver dag uanset regnskabet. Det giver korte rødder.

Examples from everyday life

Fairway efter en tør uge

Fairwayen startede ugen med 5 mm underskud. DMI giver 21 mm fordampning, og der kom 6 mm regn. Rodzonen rummer 30 mm, og du tillader 50 % udtørring.

Shortfall at start of period: 5 mm
Potential evaporation in the period (DMI): 21 mm
Crop coefficient for grass (Kc): 0.95
Rainfall in the period: 6 mm
Watering you have already given in the period: 0 mm
Plant available water in root zone: 30 mm
Permitted drying before watering: 50 %

It means: Underskuddet er nu 19 mm, og græsset har brugt 20 mm. Grænsen er 15 mm, så du skal give 19 mm.

Sandgreen med lille rodzone

Greenen startede med 2 mm underskud, fik ingen regn, men du har givet 12 mm i løbet af ugen. Fordampningen var 21 mm. Rodzonen rummer kun 15 mm.

Shortfall at start of period: 2 mm
Potential evaporation in the period (DMI): 21 mm
Crop coefficient for grass (Kc): 0.95
Rainfall in the period: 0 mm
Watering you have already given in the period: 12 mm
Plant available water in root zone: 15 mm
Permitted drying before watering: 50 %

It means: Underskuddet er 10 mm, og grænsen er 7.5 mm. Du skal give 10 mm nu.

Regnvejrsuge

Fairwayen startede med 10 mm underskud. Der kom 25 mm regn, og fordampningen var kun 12 mm.

Shortfall at start of period: 10 mm
Potential evaporation in the period (DMI): 12 mm
Crop coefficient for grass (Kc): 0.95
Rainfall in the period: 25 mm
Watering you have already given in the period: 0 mm
Plant available water in root zone: 30 mm
Permitted drying before watering: 50 %

It means: Rodzonen er fyldt op, og underskuddet er 0 mm. Vand at give: 0 mm. Der er 15 mm til grænsen.

Rule of thumb

En sommeruge med 3 mm fordampning om dagen tager ca. 20 mm fra rodzonen.

How to check the answer

Tjek med jordfugtmåleren. Viser den tørt, mens regnskabet siger vådt, så ret rodzonens vand eller Kc.