Irrigate more less frequently

Posted on behalf of Prof Moot.

Last week was hot, but not unusually so for the time of year – this is normal Canterbury summer weather. I had two experiences recently that reminded me we still have a lot to learn.

The first was coffee with a prospective PhD student. She is working in the agriculture industry and mentioned she had enjoyed doing our online 600 level course. She had learnt a lot of practical things that she could apply in her job. She had learnt from Prof McKenzie that putting 4 mm of water onto pastures every day was not an efficient use of water (which it isn’t) and tried to tell her bosses that they should be slowing down the irrigator not speeding it up. Well done, I said, you are right.

The theory is quite easy. Let’s assume you are applying 4 mm of irrigation per day every day and potential evapotranspiration is 5 mm per day. Under this scenario you are losing water from the soils above the rate of what you can supply:

4 mm irrigation applied daily  – 5 mm used in evapotranspiration daily = -1 mm/day

This is a net loss of 1 mm/day. So, your soil moisture levels continue to be depleted. But – worse than that – every day you are actually losing some of the 4 mm of irrigation applied to evaporation directly from the wet leaf and soil surfaces NOT transpiration (which allows photosynthesis) through the plant. The evaporation component does not directly contribute to plant growth. Let’s assume 1 mm of the 4 mm of irrigation applied daily (not much but it represents 25% of water applied daily) is lost to evaporation. This means of the 4 mm you applied effectively only 3 mm is available for plant growth. The stored soil moisture continues to be depleted because:

4 mm applied – 5 mm used in evapotranspiration – 1 mm lost in evaporation = -2 mm.

In practical terms you are falling behind at a faster rate than you anticipate. This is why people say “speed up the irrigator, we are falling behind”. So, over a 5 day period, of the 20 mm of total irrigation applied only 15 mm of water has entered into the soil for plant growth, with 5 mm lost to evaporation from wet plant and soil surfaces. During this time evapotranspiration totalled 25 mm (5 days x 5 mm evapotranspiration/day) so:

15 mm effective irrigation applied – 25 mm evapotranspiration = -10 mm

Effectively, over those 5 days the soil water stored in the profile has been depleted by an additional 10 mm to account for not enough water being applied to replace evapotranspiration. That 15 mm that was applied for plant growth will only meet the evapotranspiration demand for 3 days.

Now, what if we had slowed down the irrigator? (More irrigation applied but less frequently):

Assume we applied 20 mm irrigation on a single day. We would still lose 1 mm to evaporation from wet surfaces but the remaining 19 mm has gone into the soil to be transpired and used for plant growth at a later date. That 19 mm is almost 4 days of water:

19 mm applied irrigation available / 5 mm day evapotranspiration

Over a five day period you have still lost 25 mm of evapotranspiration but your soil water content has now only gone down by 6 mm, which is 4 mm less than the high frequency/low volume approach of applying irrigation daily. You have effectively gained a days water supply every 5 days by slowing down the irrigator.

I explained this to a group of farmers I spoke with before Christmas. Ironically, last Thursday I was visiting one of the dairy farmers who attended that session. He has two farms on the same soil type and he showed me his soil water profiles, which had been measured by neutron probes. On one farm he has slowed the irrigator down. The stored soil water content is falling but not as rapidly as on the other farm, which is still applying 4 mm every day. He said “I guess you were right” and made a phone call to the manager of the other farm to slow the pivot down. It wasn’t me that was right and it wasn’t a guess. This is purely science in action. If you understand the mechanisms then the management falls into place…slow down the irrigator. I asked why he wasn’t applying more water earlier – he replied he was scared to over-irrigate and lose nitrogen to leaching. That will not happen at this time of year – but that’s another story!