A Shiny app with adjustable rootzone characteristics and irrigation rules

I made this Shiny app that calculates the daily soil water balance.

The idea of the app is to change the soil conditions, specifically the rootzone depth and the field capacity, to see how changes in those parameters influence the irrigation requirement.

And the irrigation "rules" can be changed too. When will irrigation water be added? How much water will be added? What is the crop coefficient? What is the distribution uniformity of the irrigation system?

Then the soil conditions and the specific irrigation "rules" are matched to a year of weather data from a location, to see how any changes influence the amount of water required to satisfy the rules.


Estimating irrigation water requirement for different soil conditions

In previous posts, I wrote about the daily soil water balance and irrigation frequency using Bangkok (DMK) weather data, and showed how changes in irrigation "rules" can change a predicted irrigation water requirement, that time using Phuket (HKT) weather data.

Thailand conveniently has golf courses adjacent to many of its major airports, so I can imagine turf being maintained at that location, and then make the calculations using data from the airport.

Let's go to Chiang Mai. The Star Dome Golf Club sits right next to Chiang Mai International Airport (CNX).

Now I want to consider fairways, and specifically the soil type of the fairway. There are a number of advantages to growing fairways in soil, rather than in sand, and using surface and subsurface drainage, and perhaps a bit of sand topdressing, to create the desired playing surface. One of the advantages to growing in soil is a lower irrigation water requirement.

I'll use 2015 weather data from CNX. Let's imagine a fairway with a 20 cm rootzone depth, a field capacity of 40%, and irrigation at 20% to return the soil back to field capacity. With 2015 weather data, that gives an expected irrigation requirement of 718 mm.

That's a deep and infrequent irrigation regime, and with a 20 cm rootzone depth, about 40 mm will be required at each irrigation event. That's a lot of water. It would probably be more reasonable to do more frequent irrigation.

And that can save water too. For example, with that same rootzone depth and field capacity, but now irrigating at 24% to increase VWC to 30% (supplying about 12 mm at each irrigation event), the expected irrigation requirement goes down to 674 mm.

In SE Asia, it is common to sandcap fairways. For example, see this course now under construction in Thailand:

What would the irrigation requirement be for a sand rootzone at CNX in 2015? I'll keep the same rootzone depth and the same crop coefficient and distribution uniformity, just changing how much water is held in the rootzone because of the sand. I'll estimate that a fairway sand will have a field capacity of 20% (I think that is a generous estimate) and that irrigation will be supplied at a VWC of 10% to return the soil to field capacity. That gives an estimated irrigation requirement of 909 mm.

For simplicity, let's say that for the soil rootzone, the irrigation requirement is 700 mm, and for the sand rootzone it is 900 mm. Let's say this water requirement is for 10 ha of irrigated fairways. For the soil fairway condition, that gives an irrigation requirement of 70,000 m3. With a sandcapped fairway, an extra 20,000 m3 are required. Plus the energy to pump the extra water.

What happens to the irrigation water requirement after changing the irrigation "rules"?

I've shown how calculation of the daily soil water balance, matched to precipitation data, can be used to estimate the irrigation water requirement for a given set of irrigation "rules." That is, if I calculate how much water is in the soil (details about the calculation method here), carefully adding in the amount added by rainfall, and subtracting the amounts lost to drainage or evapotranspiration, I can determine when and how much irrigation is required.

And the irrigation rules are things like the quantity of water I will apply at each irrigation, the soil's field capacity, at what quantity of soil water will I reapply irrigation, the distribution uniformity of the irrigation system, and so on.

The first set of calculations I showed were for a location in Bangkok. Now let's go south, to the island of Phuket, and look at the irrigation water requirement using weather data from recent years. I got the data from the Phuket International Airport (HKT), which is just north of Blue Canyon Country Club. I'll imagine that these calculations are for a hypothetical stand of turfgrass at that location.

This is a view over the Canyon course looking north, with the control tower for HKT visible in the top left corner.


Now I will calculate the irrigation requirement using the weather data from HKT in 2015. First, I'll start with a scenario of:

  • rootzone depth at 10 cm
  • field capacity of 25% VWC
  • irrigation threshold of 12% VWC -- when the soil is predicted to drop below 12%, an irrigation event is triggered
  • each irrigation is set to return the soil to field capacity
  • the crop coefficient used to adjust the reference evapotranspiration to crop evapotranspiration is 0.7
  • the distribution uniformity of the irrigation system is 0.75

Calculating the water balance for every day of the year with those conditions, the annual irrigation water requirement is 644 mm.

That would be a classic deep and infrequent irrigation regime. For that same location and same weather data, what happens if I change to a light and frequent approach? Now I'll irrigate at 15%, rather than at 12%, but I will add only enough water at each irrigation to reach 20% VWC in the top 10 cm, rather than 25%. In this case, the annual irrigation requirement drops to 620 mm.

What might happen if I start using a (or use an improved) soil surfactant? I could reasonably expect that the spatial variability in soil water content would be reduced, and that the soil would be easier to rewet after drying. I can go back to the original deep and infrequent rules, but now with the surfactant use I will let the irrigation threshold drop down to 10%, instead of the more conservative 12%. With the surfactant, I think that is a reasonable and safe adjustment. Now what happens? The irrigation water requirement drops from 644 mm to 605 mm.

What happens if I can get the roots to grow a little deeper? If I then increase the rootzone depth from 10 cm to 12 cm, the irrigation water requirement goes from 620 mm down to 569 mm.

Here's a way to make a substantial drop in the irrigation water requirement -- improve the distribution uniformity of the irrigation system. If I improve the DU from 0.75 to 0.8, while keeping the other rules as in the previous scenario, the irrigation water requirement goes from 569 mm to 533 mm.

And if I then go back to frequent irrigation rules, in this case irrigating at 15% and adding water to increase the top 12 cm to 20%, the irrigation water requirement is 529 mm.

Simulating irrigation frequency at the world famous "snake" course

The "snake" course, and simulation using the daily soil water balance

Many of you will have seen the Kantarat Golf Course when flying into Bangkok. Maybe you've played it. It's a cool course, set between the two runways at Don Mueang International Airport in Bangkok.

It is commonly called the snake course, and I can confirm there are a lot of snakes out there.

image from c1.staticflickr.com

And then there are the planes, and the crossing of active taxiways.



The most common problem with irrigation water quality is high salinity, and the solution to that problem is adjusting the quantity of water supplied. At the end of yesterday's seminar, I switched from talking about water quality, and discussed the application of the daily soil water balance in managing irrigation water quantity.

I used the snake course as a hypothetical location, because I had a set of daily data from the weather station at Don Mueang (DMK).

More about irrigation frequency

I've written previously about whether it is better to do deep and infrequent irrigation, or whether it might actually be better to irrigate frequently in small amounts. I applied the daily soil water balance to work through this for a location at DMK.

Let's say we are growing grass at DMK and have a 10 cm rootzone depth and then the weather happens as it did every day at that location in 2015.

I'll have some plan of how I'm going to irrigate, too. Let's say there is a field capacity of 25%, and I expect the grass may wilt when the volumetric water content (VWC) is less than 10%. I will try to irrigate to keep the soil from dropping below 12%, and every time I irrigate, I will fill the soil back to field capacity. When I do that, with the details as shown here, for example using a crop coefficient (Kc) of 0.7 and a lower quartile distribution uniformity (DULQ) of 0.75, I can then simulate the soil water content day by day through the year. I do that by stepping through each day of the year, with the evapotranspiration and precipitation as it happened, adding irrigation as required by the rules I've set. Doing that for 2015, the irrigation requirement is 1011 mm and the median VWC through the year is 19.7%.

image from c1.staticflickr.com

I can also simulate the soil water content and irrigation required for a different set of rules, but for the same soil and weather. I did that, for those same 2015 weather data, now irrigating at 14% rather than at 12%, but instead of supplying enough water to raise the soil back to field capacity, I only add enough to increase the soil water to 18%. When I do this, the irrigation requirement drops to 970 mm, and the median VWC goes to 15.5%.

image from c1.staticflickr.com

I checked this for 2016 data, and the results were similar: a total 949 mm of irrigation required and median VWC for the year of 20.2% using deep and infrequent rules, 889 mm irrigation and a median VWC of 15.6% with light and frequent irrigation.

image from farm1.staticflickr.com

image from farm4.staticflickr.com

My presentation on irrigation water quality

Yesterday I taught a seminar about irrigation water quality.

Here are some links related to that presentation.

After 28 days, grow-in and salinity differences


I've been growing grasses in a plastic house with a lot of help from colleagues at the Thailand Institute of Scientific and Technological Research (TISTR). The idea was to see how these grasses grow in after being planted as stolons, and to see what happens when salt is added in the irrigation water. I'll be discussing this experiment at the field day in Chonburi next week.

The picture above shows the grasses that receive the irrigation with 330 ppm total dissolved solids (TDS), 28 days after planting. The seashore paspalum looks the best, and the nuwan noi manilagrass has grown-in almost as fast. The hosoba korai, which is a beautiful grass once established, still hasn't covered much of the pot.

Another thing I've found interesting is measurements of salinity in the soil with the new TDR-350. All the pots are supplied with the same quantity of water. But different sets of pots get different amounts of salt in the water.


The soil salinity in these pots is changing depending on which irrigation water is applied. That's just as expected.

For more about the TDR-350, see this webinar.


High expectations


I've rarely been so excited to read an article. Last week when I saw Energy use and greenhouse gas emissions from turf management of two Swedish golf courses, by Tidåker et al., I immediately dropped what I was doing and read it.

If you've talked with me about turfgrass management sometime in the past 18 months, our conversation may have touched on differences in energy use, and the difference in carbon emissions, caused by differences in grass selection and maintenance practices. In fact, this is one of the topics Dave Wilber and I discussed as part of our wide-ranging conversation during episode 14 of the Turfgrass Zealot Project. I don't know how to make these calculations yet, but finally with this article I've read something that provides the calculations, and that I can study so I can figure out how to do this myself.

Gelernter et al. wrote in 2014 about quantifying sustainability on golf courses. We suggested measuring and tracking the annual:

  • quantity of fertilizers applied
  • quantity and toxicity of pesticides applied
  • quantity of water used
  • fuel volume
  • labor hours
  • electricity used

One can keep track of those quantities, together with the associated costs, and from that one can check the efficiency of the operation. These quantities also serve as some of the basic data requirements for the GEO OnCourse program.

But the quantities we wrote about in the GCM article are all different: kg of N, kg of fungicide, L of water, L of diesel, kWh of electricity. By expressing all the turf maintenance activities in units of greenhouse gas emissions (expressed as CO2 equivalents) or energy use, one then has a single number for the entire course, or for an area of the course, or per square meter, that can be used to compare to other courses next door or around the world. And the use extends well beyond comparisons to other golf courses; one can use these units to compare the maintenance of a golf course to anything that has greenhouse gas emissions or energy use.

I had high expectations for the article, and I wasn't disappointed. The authors described the fertilizer rates, topdressing rates, water use, mowing frequencies, and much more, for the two courses, and then expressed those units in GHG or energy use. N rates were up to 22 g/m2, as were K rates (I think the rates for golf course turf in Sweden should usually be less than reported in the article -- using precision fertilization, or temperature-based growth potential and MLSN, will lead to lower recommended amounts of fertilizer). Sand topdressing on greens was about 10 mm/year. Irrigation of greens was about 300 mm/year. Mowing of fairways was about 85 times/year, and greens were mown about 180 times/year.


I think this is fascinating because one can consider Sweden to have relatively low inputs. If you're familiar with golf course maintenance in a tropical environment, let's say in Phuket, you might expect fairways to be mown more than 150 times a year, greens more than 300 times, about double the fertilizer, and more than twice the water use. Now imagine what happens when comparing irrigated vs non-irrigated rough? Seashore paspalum wall-to-wall vs. manilagrass? A 60 ha sandcapped golf courses vs. one with drainage and 2 cm of sand topdressing? Overseeded vs. not? The differences in energy use and greenhouse gas emissions will be huge.

What did Tidåker et al. find in their analysis? The entire paper is worth a careful study, but in summary they found mowing was the most energy-consuming activity, and mowing together with the production and application of fertilizers (especially N) contributed the most to greenhouse gas emissions. They suggest:

Appropriate measures for reducing energy use and carbon footprint from lawn management are thus: i) reduced mowing frequency when applicable, ii) investment in electrified machinery, iii) lowering the mineral N fertiliser rate (especially on fairways) and iv) reducing the amount and transport of sand for dressing. Lowering the mineral fertiliser rate is of particular importance, since GHG emissions originate from both the manufacturing phase and from N turnover after application.

Jason Haines has some interesting reads about how turf condition can be improved while at the same time reducing inputs:

More on the daily soil water balance


How much irrigation is required at a particular location? That is an interesting question, and one gets widely different answers depending on the method used for calculation.

The standard method involves taking the consumptive water use and subtracting the effective rainfall. That seems like it would be an effective way to calculate how much irrigation is required. However, that method doesn't explicitly consider the depth of the rootzone. Because managed turfgrass has a relatively shallow rootzone caused by low mowing heights, I think it makes more sense when making the calculations for turfgrass to find the irrigation requirement by using a daily soil water balance.

For a little background on this, see:

Last week I made calculations of the daily soil water balance and from that calculated the irrigation water requirement.

As an example, this is the volumetric water content (VWC) of a 15 cm deep rootzone at Fukuoka this year, assuming irrigation was supplied when the soil would drop below a VWC of 10%, and that the quantity of irrigation supplied at each irrigation event was enough to fill the soil to field capacity.


To get the quantity of water required for irrigation, one adds up the water required for each of the irrigation events. By using the weather data for a particular location, and by adjusting the rootzone depth and field capacity and irrigation rules for that location, the daily soil water balance gives a value for the irrigation requirement that should be close to the real one. By irrigation rules, I mean how much water is applied at each irrigation event, and what the threshold VWC is for applying irrigation.

For example, I calculated this for a 15 cm rootzone at Sapporo for the months of July, August, and September, using weather data for the past 10 years.


When I calculate the daily soil water balance and then add up the amount of water required as irrigation for a 15 cm rootzone depth with the rules as shown on the chart, only 2 of the 30 months on the chart had an irrigation requirement of zero. Those are September 2015 and September 2016. The month with the highest irrigation requirement was August of 2008, with 82.7 mm; July 2007 had an 82.3 mm irrigation requirement. The median irrigation requirement for those summer months at Sapporo for the past 10 years was 42.5 mm/month.

After I shared some charts of the VWC calculated from a soil water balance, @turfstuf asked me about showing the annual quantity of plant water use together with the quantity of water required as irrigation. Those charts look like this for Sapporo, Fukuoka, and Naha:




It rains a lot in Japan, so for a normal year, a large amount of the plant water requirement can be supplied by rainfall. That's why the irrigation requirement and the plant water requirement lines are separated. The gap between the lines represents the amount of water that is supplied by effective precipitation.

The rootzone depth and the irrigation rules will also have an effect on the quantity of irrigation water required. The previous charts were for a 15 cm rootzone depth. This shows the difference between a 10 and 20 cm rootzone Fukuoka for the past 10 years.


What about locations with less rainfall? Have a look at this chart with data from Stovepipe Wells in Death Valley, where it doesn't rain much, and you'll see the irrigation requirement by the daily soil water balance method is almost the same as the plant water requirement.

Stovepipe wells

Gelernter et al. used a daily soil water balance in their analysis of water use on golf courses in the United States. This approach has many applications. For example, one can predict an irrigation requirement given past weather data. One can also compare the actual irrigation amount to the predicted amount. And one can adjust irrigation rules and other parameters in the daily soil water balance calculations to find what the change in irrigation requirement would be if those adjustments were made.

Daily versus monthly calculations of ET and irrigation requirement

I showed how weather data can be used to calculate a daily soil water balance. One can adjust the rootzone characteristics, and the timing and amount of irrigation, so that the calculations are representative of what one wants to know.

By keeping track of what the soil water content would be on each day, given the actual weather conditions, and given the water holding capacity of the specified rootzone, one can find how much irrigation water would be required.

I've also made calculations using the standard method, which takes the evapotranspiration (ET) and subtracts the effective rainfall. I've used this method before to make calculations, and it made sense to me, but I've realized that this method doesn't account for rootzone depth. For turfgrass, one should probably adjust the effective rainfall calculation for each site based on the rootzone depth.

I wondered if these methods give a similar result in predicting the irrigation requirement. I had daily data from Sapporo from 2013 to 2015, and I also got the monthly averages or totals for the same time period. I've just made some calculations to find out.


I looked at the months from April to October in each year. That's a total of 21 months.

For the ET, the result is almost the same whether it is calculated daily, and summed for a month, or whether one calculates ET using the monthly data.


For the irrigation requirement, there is not a consistent agreement. I made these calculations based on an approximation of a loam soil with a 10 cm rootzone depth, a field capacity of 40% (by volume), with irrigation supplied to return the soil to field capacity when soil water content would drop below 20%.


I've got some more calculations to make about this. The standard method seemed pretty good to me until I started making the daily calculations.