Review: The effect of pot volume on the performance of pot irrigation system (Al-Mohammed, 2012)

Comment

The description of the set-up lacks some details.

Considering that the author measured the “surface wetting edge”, the more dense, unprepared soil in which the holes for the clay pots were dug has a strong chance to influence the size of the area around the pots that gets irrigated but this potential influence is not mentioned in the paper.

The author calculates the decrease in yield irrigated with small pots based on the yield of crops grown with large pots for irrigation. It is even more significant to take a look at the increase of yield gained by irrigating with large pots compared to small pots. Compared to the yield and water use of the smaller pots’ set-up PSI₂ the increase in yield and water use in set-up PSI1 looks as follows, based on the data presented in tables 3 and 4:

Since the title of the paper reads “The effect of pot volume on the performance of pot irrigation system” the author should have outlined and discussed these effects to a greater extent, especially the effect and degree of increase in yield from large pots compared to small pots. This increase means for farmers that investing in larger pots could lead to a higher income in the long term.

Of importance for farmers is also the result that cucumbers grown with large clay pots for irrigation produced 130% more yield with less than 30% increase in water use compared to cucumbers grown with small pots for irrigation.

The authors makes two fundamental mistakes in his “conclusions”:
1.)
“The difference in crop water requirement under PIS1 and PIS2 is significant, higher crop water requirement is obtained under PIS1.” [E96]
“Crop water requirement” is a misleading term since what is measured here is not the water explicitely used by the crop but the amount of water used for irrigation. It is not the case that 100% of the irrigation water is consumed by the plant. Parts of the water, e.g., wet the soil, seep away or evaporate.

It goes without any further argument that a clay pot with a higher volume and a larger surface area provides more water to the soil and thus creates a higher wetting radius (as the author also shows in fig. 6) than a smaller pot of the same material and firing temperature. It is not necessarily the crop that requires more water, it is the pot that provides more water based on physical laws. In addition the pots in this experiment were continuously supplied with water which also increases the amount of water seeping into the soil since the pots were always filled.

As a consequence “crop water requirement” should be replaced with “water use”.

2.)
“PIS2 is a conservation irrigation system, which saves between 22% and 41% water when compared to PIS1.” [E96]
This claim is misleading, too. The experimental set-up labeled “PIS2” used smaller pots compared to the set-up labeled “PSI1”. In both systems the pots were continuously filled with water up to their necks. Of course the seepage rate of a smaller pot with a smaller surface area is lower than the seepage rate of a pot almost fourth the size when both pots were made of the same material and fired by the same temperature. So yes, less water was provided to the crops by the smaller pots, but this must not be confused with “saving” water, because the lower water use resulted in less yield – totally as well as relatively. The PIS2 set-up with the smaller pots did not “save” any water, as the author writes, the pots deliverd less water to the plants and could not provide enough water to fulfil the crop’s water need to produce a higher yield.In other words: If both set-ups were irrigated with the same amount of water the larger pots would have provided a higher yield. Table 3 clearly shows that the water use efficiency of the large pots is much better for all three crops than that of the small pots. Knowing this, the author should have recognized that there is no water “saving” by using the small pots. He actually points this out on page E95: “This means that these crops irrigated by the PIS1 require less water to produce a higher yield per unit applied water.” but still draws this erraneous conclusion.

The author does not differentiate between results and conclusions. He basically summarizes the results in his conclusions section.

Within the replications of PIS1 as well as of PIS2 some results concering yield and water use differ strongly but the author does not address these differences. Since the experiment was conducted in a greenhouse with identical soil, water quality and weather conditions for all the pots and crops, these differences are unexpected and would have needed an analysis and discussion of the potential underlying reasons.