There is software such as CROPWAT to estimate crop irrigation requirement. But DSSAT has more detailed process-based crop models, and I was wondering how can we derive crop water requirements and irrigation requirement using DSSAT.
Here is some work relevant to your post, hope you like them...
DSSAT determines daily irrigation needs, based on read-in values or automatic applications based on soil water depletion. This will be done with the soil water module. With respect to CROPWAT it uses Modified PM equation and calculates the crop water requirement (ETc) and irrigation requirement including scheduling. Please find attached one of my paper on CROPWAT for your reference
I have mainly used APSIM models. It is a long time since I used DSSAT. Usually you can run the models with a 'water non-limiting' setting and set the output to give you the cumulative daily water use (ET). Running single-year simulations using historic weather data and your soil parameters will give you an average total crop water use and a measure of its variability across years. Set the soil water at sowing to the drained upper limit (DUL).
The irrigation requirement is a little more complicated. Basically, you use your historic weather data and do single-year runs with the model with a definition to trigger irrigation at a given soil water deficit, but then it gets messy. I think the best thing to do is to record the soil water deficit at the time each irrigation is triggered. This give you the irrigation requirement in terms of the water that is being put into the soil. HOWEVER, then you need to take into account the irrigation application efficiency which will vary with soil type, surface conditions and application method (and possibly the soil water deficit). That will then give you a measure of the irrigation water you need to be able to supply to the field.
Two complications in this are (i) choosing the most appropriate irrigation deficit and (ii) accounting for the soil moisture available at sowing.
The appropriate irrigation deficit that you choose to use will depend on whether you are trying simulate a particular scenario (For example, the current local practice or the current proposed best practice defined by some organisation) or whether you are trying to optimise the system. In the latter case, repeat the runs with a range of different deficits and choose the one which gives you the best result. You need to decide whether you are optimising yield, yield variability or some other parameter.
The question of soil moisture at sowing can be dealt with in a variety of ways. In some areas you can assume that the soil water status at the harvest of the previous crop is at the crop lower limit (CLL). In this case you can start your simulation runs from that point and let the weather input data generate 'natural' variation for the starting point of the crop simulation. An alternative is to run your simulations with a range of soil water content at the sowing.
Best regards,
Steve Milroy.
@ Stephen P Milroy. Thank you sir for your excellent detailed answer.
I am trying to optimize the irrigation water input (water productivity) in DSSAT instead of yield.
If I run the DSSAT with automatic irrigation scheduling it will qualify for the "water non limiting criteria". CROPWAT by FAO computes irrigation requirement, is it similar to DSSAT automatic irrigation or DSSAT will be more accurate, do you have any idea about this sir?
So, I think soil water will play an important role in deciding the optimized irrigation schedule as well as irrigation technology, am I right?
A good person to ask on the relative accuracy of the different models and simulation environments would be Prof Senthold Asseng of Florida University. He is very knowledgable and a helpful person. I suspect they will be similar.
Optimization of the irigation schedule will be influenced by the irrigation method, soil surface conditions, soil infiltration and drainage characteristics, soil water holding capacity, and of course, in season rainfall and its variability.
All the best,
Steve
@ Stephen P Milroy Okay sir, I will contact Prof Senthold Asseng.
Regards
Jyoti
i think aquacrop is good for net irrigation requirement and generation of irrigation schedule, but cropwat is better in terms of ETc and Effective rainfall estimation for any location
you can use most recent version of DSSAT (4.7.5), where in simulation options you can use automatic irrigation to irrigate at different levels of depletion from soil available water, or you can use irrigation based ET.
In my opinion multi model comparison will be most appropriate strategy in this regard.
what are the basic equations involved in crop yield computation in DSSAT model ?
@Jyoti Singh
There are two equations are involved in DSSAT to estimate ETo & ETc
1. Penman Monteith &
2. Prisley Taylor
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