Poor soil moisture levels and lack of rain on the Darling Downs is leading many to revisit planned planting decisions for the upcoming season. This article was created to give Cotton growers a feel for how much irrigation might be required under different rainfall scenarios, row configurations and stored soil moisture on the black vertisols typical of the Darling Downs. Lighter soils will require more irrigation.
Useful links, along with the assumptions used in the creation of the tables below, will be provided further down in the article.
Click on the link below for a PDF version.
- ¹ "Water required" is the moisture needed from irrigation and rain to attain full yield potential. It does not include water lost to inefficiencies in irrigation.
- ² Furrow irrigation is assumed to be 80% efficient and therefore for every 1ML of irrigation delivered to the field, 1.2ML of irrigation water will be used.
- Higher water use on the first irrigation was not modeled which will result in a slightly lower number of irrigations required when starting with lower initial soil moisture.
- A high clay content black vertisol soil.
- An estimated Plant Available Water Capacity (PAWC) based on our experience of black vertisol soils on the Darling Downs. If a soil is lighter, more irrigations will be required.
- PAWC to a depth of 90cm. There is potention for Cotton to extract moisture to levels as deep as 1.2 and 1.5m so the tables may overestimate irrigation required in some circumstance.
- Rooting capable of extracting full PAWC is assumed to reach the 90cm depth for the full profile under all the configurations. There will be circumstances where Single or Double skip will not be able to fully extract in the "skip".
- 850mm of moisture is required for full yield potential. Cotton requires between 800 and 850mm of moisture.
- The number of irrigations have been rounded to the nearest whole number using mround in excel. eg. 2.4 irrigations will be rounded down to 2 and 2.5 irrigations are rounded to 3.
The best place to start for more detailed information regarding irrigations and water use is WaterPAK - CRDC.
A useful fact sheet describing the decisions for Semi Irrigated cotton can be viewed here.
A good breakdown for the decisions revolving around row configurations can be viewed here
If you would like a more precise understanding of moisture available in the your soil, Black Earth Agronomy can provide measurement using an EM38.
The information provided above is intended as a guide only. There are a number of situations and circumstance where the assumptions and information in this article will not hold true. Consult your agronomist for a better understanding of requirements for your particular farm and fields.