Review: Seepage Rate from Ceramic Pitchers under Positive and Negative Hydraulic Head (Abu-Zreig et alii, 2018)

Table of Contents

Study
Objective of the study
Overview
Contents
Description of the experiments
Results
Conclusions
Comment

Study

Title: Seepage Rate from Ceramic Pitchers under Positive and Negative Hydraulic Head
Authors: Abu-Zreig, M., Zraiqat, A., Abd Elbasit, M. A. M.
Type of Study: Experimental
Journal: Applied Engineering in Agriculture, Vol. 34, Issue 4, 2018, 707-714.
Open Access: No. 68 USD
Link: https://elibrary.asabe.org/abstract.asp?aid=49539&t=1&redir=aid=49539&confalias=&redir=[volume=34&issue=4&conf=aeaj&orgconf=aeaj2018]&redirType=toc_journals.asp&redirType=toc_journals.asp

 

Objective of the study

“The objective of this study is to explore the possibility of using ceramic emitters for irrigation in small and large scale using baked clay pots.” [708]

 

Overview

Type of experiment: Lab
Subject: Seepage rates of pitchers
Country: Jordan
Place: not given
Timespan: uncertain, 4 days?
Crops:
Soil Type: “Tottori sand dune soil” [708]: Sand 96.1%, Silt 0.4%, Clay 3.5%
Number of pitchers: 7
Parameters analysed: Seepage rate in the atmosphere, seepage rate of pitchers buried in soil, seepage rate of pitchers under positive and negative hydraulic head

 

Overview

The paper is structured into four sections plus references.

[Introduction]

Materials and Methods
Seepage measurements
Seepage Rate in the Atmosphere
Seepage Rate of Pitchers Buried in Soil
Seepage Rate of Pitcher Under Positive and Negative Hydraulic Head

Results and Discussion
Seepage Rate under Positive and Negative Head

Conclusions

References

 

Description of the experiments

Pitcher characteristics
The pitchers in the experiment were handmade by local producers. [708] In addition to the data provided in table 1 the authors mention an average wall thickness of ~ 6 mm, an average bulk density of the material used to construct the seven pitchers equal to 2167 kg m-3 and an average material porosity equal to 38%. [708]

Pitcher Volume (ml) Surface area (cm2) Height (cm) Maximum outside diameter diameter (cm) Neck and base diameter (cm)
P-1 1.281 713 25.8 14.0 5.6
P-2 1.220 699 25.4 14.0 5.4
P-3 1.279 710 26.5 14.1 5.4
P-4 1.286 710 25.9 13.1 5.0
P-5 1.330 730 26.6 14.1 5.5
P-6 1.366 737 26.6 14.2 5.7
P-7 1.197 691 25.7 13.2 5.6

 

Experimental Set-up
Seven ceramic pots “simulating ceramic emitters” [707] were used in the experiment.

Seepage Rate in the Atmosphere
To “estimate the free seepage rate of the pitchers in the atmosphere” [708] several experiments were conducted “inside a chamber under controlled temperature and humidity” [708].

“The volume was recorded at 24 h interval for at least four consecutive days.” [708] The “loss of
water in the pitchers was measured by measuring the refilling volume of each pitcher.” [708]

“The procedure was repeated for each climatic condition in which temperature was varied from 10°C, 15°C, 20°C, 25°C, and 30°C whereas the humidity was kept constant at 40%.” [708]

A “Small steel circular pan was placed in the chamber to determine the pan evaporation, Ep, under each climate condition.” [708]

Seepage Rate of Pitchers Buried in Soil
It seems that the same seven pitchers from the first experiment were used for the second one as well. They “were placed in the middle of a 30-L plastic soil pot then filled with air dry Tottori sand dune soil (sand= 96.1%, silt=0.4%, clay=3.5%). (…) The pitchers were filled up to their neck with water and left in the chamber for 24 h. After that, the pitchers were refilled again measuring the seepage rate in mL day-1. The weights of soil buckets were also recorded every 24 h to measure the
rate of evaporation under various pitchers.” [708]
“(…) the soil volumetric water content was recorded at 30 min interval.” [709]

Seepage Rate of Pitcher Under Positive and Negative Hydraulic Head
“Constant positive or negative hydraulic head relative to the pitchers mouth was applied by increasing or decreasing the Marriott bottle level relative to pitchers mouth. (…) Each experiment lasted for 2 to 3 days in which seepage volume under each head was measured every 12 h to ensure steady state conditions under each head value..” [709]

Results

“(…) the seepage rate of pitchers was linearly and significantly related to the hydraulic conductivity (Ks) of pitchers wall material as shown in figure 6.” [709-710]

“Figures 7 and 8 confirms that seepage rate of ceramic pitcher when buried inside soil were about two-fold higher in average compared to that in the atmosphere.” [710]

“A plot of seepage rate of pitchers inside soil versus that in the atmosphere, shown in figure 8, revealed a linear and significant (P<<0.01) relationship (Qsoil = 1.9 Qair) with coefficient of determination R2 = 0.98. These results are useful because it enables us to estimate the expected amount of water seepage of pitchers before installing it inside the soil with simple test in the atmosphere.” [710]

“(…) seepage rate was found to decrease steadily with an increase in the soil moisture around emitters (…).” [710]

“Soil moisture decreased with evaporation thus increasing soil suction pressure and therefore increases seepage rate from pitchers.” [710]

“Seepage rate from ceramic pitchers as expected increases with an increase in the hydraulic head. But the interesting result is that seepage rate occurred even under negative head. (…) The implications are that ceramic emitters can supply water to plants without creating a positive pressure in the irrigation line and one can also control how much water is released by controlling the hydraulic head of ceramic emitters. (…) Therefore, the system can be successfully utilized for deficit irrigation where water is released at smaller quantities just below crop-water requirements. In addition, eliminating the need for pressure will eliminate the energy required to operate the irrigation lines.” [712-713]

 

Conclusions

The authors mix results and conclusion in the results section and the conclusions section consists of a summary of the results and a single brief conclusion:

“Seepage rates from pitchers increased by about three-fold under constant head compared to that of variable head conditions and also increased by two-fold when pitchers buried inside soil compared to that in the atmosphere. In addition, seepage rates from pitchers have increased with an increase in the evaporation rate or a decrease in the soil moisture around pitchers’ wall. We have found also that seepage rates of pitchers can occur even under negative hydraulic head indicating the possibility of using pitcher irrigation systems for deficit irrigation.” [713]

 

 

Comment

This study displays several serious flaws in regards to the experimental set-up, the interpretation and presentation of the results, and the conclusions.

 

Seepage rates

The authors claim that “The free seepage volume of pitchers can be correlated to seepage volume through soil under field conditions.” [708] They do not quote or reference any studies but base their claim on their own small scale experiment: “Figures 7 and 8 confirms that seepage rate of ceramic pitcher when buried inside soil were about two-fold higher in average compared to that in the atmosphere.” [710]

These claims have multiple flaws:

    • The experimental set-up of the authors is not suitable to allow this kind of claim. The seepage rates of the pitchers buried in soil were measured in a tiny controlled chamber (see fig. 3) lacking various relevant parameters constituting “field conditions”. The pots were also buried in much too small plastic buckets (see fig. 3) to determine their natural seepage rate “in the field” (keyword: saturated hydraulic conductivity of the soil).
    • Another problematic feature of the lab experiment concerning the seepage rate in the atmosphere is that the pots stood in bowls – metal and plastic as it seems and of different sizes – in which the seepage was collected, see fig. 2 on page [709]. Based on the photograph it looks as if at some point during the experiment the bottom part of the pots must have stood in water. Also the collected water in the bowls evaporates which influences the seepage of the pots.
    • The pitchers were buried in soil consisting of 96.1% of sand. This is a very unrealistic scenario for any farmer (see the objective of this paper given by the authors) and the authors do not provide any reason for their unusual choice.
    • The photograph of the clay pots buried in the small plastic buckets (fig. 3) seems to display at least two different soils in the buckets based on a very light and a dark colour. If the different colours are based on wet and dry soil then the wetting patterns would be very irritating: The soil in the bucket in the back left corner would be dry around the pitcher but completely wet at the edge of the bucket and the soil in the middle bucket to the left would be completely dry while the soil of other buckets would be completely wet. The description of the set-up does not mention a time delay for selected buckets. So the questions remains how the different colouring of the soils in the bucket can be explained.
    • The value “about two-fold higher” is incorrect. The average increase is not around 100% but it is 88%.
    • When presenting averaged data it is still important to look at the individual data, especially if you work with small data sets like seven pots. Only two pitchers display a 99% and a 106% increase, the remaining five pitchers display increases of only between 72% and 89%. The authors did not look at the individual differences between the increases of seepage rates which vary between 72% and 106%. While statistically it is always possible to calculate average numbers, it is critical to do so with too small data sets and even more critical to use the results based on such a small data set to draw generalizing conclusions.

 

The authors write: “A plot of seepage rate of pitchers inside soil versus that in the atmosphere, shown in figure 8, revealed a linear and significant (P<<0.01) relationship (Qsoil = 1.9 Qair) with coefficient of determination R2 = 0.98. These results are useful because it enables us to estimate the expected amount of water seepage of pitchers before installing it inside the soil with simple test in the atmosphere.” [710]

    • This claim is based on an analysis of just seven pots which displayed highly individual data that was averaged in an experimental set-up with multiple issues (see above). The authors conclude from a data-set like this to an overall ability to apply their results to a general use in the field.

When proclaiming a correlatability of the free seepage volume of pitchers and the seepage volume through soil one of the authors is likely referring to his own similar study from the year 2004 which displayed multiple serious flaws, too, see the review here.

 

A lost objective

While the authors claim that “The objective of this study is to explore the possibility of using ceramic emitters for irrigation in small and large scale using baked clay pots.” [708] this objective is not addressed anywhere else in the paper again so that the question remains open if it is possible to use ceramic emitters for large scale applications or not. Even worse on page [709] the authors state: “The behavior of these pitchers is similar to ceramic emitters that can be commercially manufactured in smaller size.” This statement rules out any large scale applicability but the authors do not seem to realize that they undermined their own objective with this sentence.

The pitchers the authors used in this study had a very small volume of only a little over 1 litre combined with a hight of around 26 cm. These long, rather thin pitchers are much too small for any beneficial use in the field for several reasons, e.g., they would increase irrigation labour dramatically and can only produce a very small horizontal wetting radius which would restrict the planting radius as well as the number of plants around a pot. They are especially not applicable in a “large scale using” context the authors claim on page [708].

The authors claim that “In this study, we used large size emitters, called pitchers, because they are easier to study and commercially available from the ceramic pottery sellers.” [709] But the pitchers had a max. volume of only 1.366 liters which is not “large size” at all in terms of field application. Also “easier to study” is not a legitimate argument in a scientific study.

If the authors were serious about their objectives they should have analysed pitchers that are common in field applications or that are at least commonly represented in field and case studies concerning clay pot irrigation applications. In 2018, the year of the publication of the study, over 100 scientific publications about clay pot irrigation plus multiple case and field studies were available online, the majority in open access.


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