FAQ's   (frequently asked questions)   and   answers

Since the opening of the website a number of questions about soil problems, reclamation, drainage systems, irrigation, land and water management, and statistics have reached me.                            
 
Sometimes I was unable to provide an adequate answer.

However, there were questions I could categorize and answer collectively through generalized texts that were made for the occasion.

More papers can be found on the "articles" page and the "reports &cases" pages.

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The following 26 questions were frequently asked.

They are arranged by subject: (A) Drainage,   (B) Irrigation,   (C) Soils,   and (D) Statistics.                                                                                                                                                                

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(A) Drainage subjects

   
figure drainage design
            Depth of the watertable (X) and crop production (Y), field data
    1 - What is waterlogging? (This is probably the most frequently asked question}                            

    In agriculture, waterlogging means that the soil is considered too wet to cultivate crops profitably (except lowland rice).
Waterlogging can be stagnation of water on top of the soil surface and/or the presence of a high water table inside the soil (at a depth of say 0.7 m on average).
Surface waterlogging can be seen as a flood control problem or a surface drainage problem.
Subsurface waterlogging can be tackled by a subsurface drainage system.
As an alternative one may opt for another kind of land use.
More information can be found in an article on Drainage Criteria and in a paper on world wide cases of Drainage Systems.
See also Wikipedia (watertable control).

 
2 - What kind of drainage systems are applicable under what conditions?

    It is difficult to give a generally valid answer to this question, but the paper on surface and subsurface Drainage systems gives a summary of practical experiences with different methods under varying agro-climatologic conditions.
See also Wikipedia: Drainage sytems, or: WaterWiki (Land drainage).

3 - How can I calculate drainage discharge needs and irrigation/leaching requirements for salinity control using water and salt balances without resorting to simulation models like SaltMod and SahysMod?

    The answers, including the water balance of interceptor (interception) drainage, may be found in a lecture note on Drainage for Agriculture presenting Drainage and Hydrology/Salinity, Water and Salt balances.
See also Wikipedia: Agricultural hydrology, and: Salinity control, or: Waterwiki.

4 - What is the influence of land slope on the spacing of subsurface drains?

    The answer is: for drain spacing calculations the slope of the land has little influence. For this purpose, sloping lands may considered as if they were flat.
This is a paper with mathematical explanations

 
5 - How can hydraulic conductivity (permeability) of the soil be measured and determined?

    The measurement of hydraulic conductivity can be done in the laboratory or in the field, above or below the water table, using small scale (auger hole method, infiltration method) and large scale methods (e.g. using drainage data in existing experimental fields or in farm land).
A full description is given in the chapter "Measurement of hydraulic conductivity of soils".
See also the table of contents on line.
For well pumping tests to determine aquifer hydraulic properties see the WellDrain model (see the software page).

 
6 - Drain spacing equations for horizontal tile/pipe drains are well known. Can subsurface drainage also be done by (tube)wells and how can the required spacing be calculated?

    The calculation of well spacings for subsurface drainage can be done with the WellDrain program (see the software page).
        The theory behind it is discussed in the paper Well spacing equations.
For well pumping tests to determine aquifer hydraulic properties see the WellDrain model (see the software page).

 
7 - What is the optimimum depth of the water table and how can we develop suitable drainage criteria to design a subsurface drainage system
    This is the lecture note on Agricultural Drainage criteria.
The table of contents can be seen on line.
See also Wikipedia: Drainage research and: Watertable control, or: Research in drainage.

 
8 - What envelope materials (filters, wraps) are used for pipe drains?

    This is a pdf file made of scannned pages on the subject taken from an ILRI publication.

 
9 - How is the river and surface water pollution tackled in The Netherlands?

     I am not a lawyer nor a specialist on pollution with organic compounds. Yet I made a summary of the Dutch "Law Pollution Surface Water" to the best of my ability. This is the summary.
Further a powerpoint presentation of the summary ("summary of the summary") can be downloaded or viewed on-line.
 
figure furrow irrigation
                              Controlled drainage
Free of obligations on either side
 

(B) Irrigation subjects

     
figure Okavango
Women high up the the Andean mountains
are good irrigators. See this paper.  
 
 
figure furrow irrigation
"Over the surface irrigation" with furrows

 
Of the almost 300 million ha of irrigated land in the world, 94% is irrigated by "over the surface irrigation" methods, see Wikipedia

     
10 - Is there global information on irrigation statistics, management, and environmental impact?

      This is a paper giving information on these irrigation subjects.
See also Wikipedia on the statistical, managerial, and environmental aspects of irrigation.
 
 
11 - What about water management and soil salinity in alluvial fans or inland delta's?

      On this website there are 3 papers on water management and soil salinity in alluvial fans (inland delta's):
1. Flood recession cropping in the molapo's of the Okavango Delta, Botswana.
2. Irrigation projects in the alluvial fan of Punata in Bolivia.
3. Water management and soil salinity control in the alluvial fan of Garmsar in Iran.
 
 
12 - Is irrigation high up in the mountains feasible?

      This is a paper discussing the importance of irrigation at high altidues in the Andean mountains.
 
 
13 - What is spate irrigation, how effective is it, and what is its potential?

      This article discusses the problems and potential of spate irrigation in the NWFR province of Pakistan.
 
 
14 - What is tidal irrigation and how does it work?

      This is a paper in tidal irrigation of date palms in the Abadan Island in SW Iran.
 
 
15 - What does the irrigation in Egypt look like after construction of the Aswan High Dam?

      Please consult this report on irrigation improvement along the Nile river in Egypt.
 
 
16 - What is the development of river salinity in downstream direction when, in arid and semi-arid regions, the salty river water is used for irrigation of agricultural land and the drainage flow and effluent are returned to the river, and what are the effects on soil salinity and how can eventual problems of salty soils be tackled?

      This is a paper on river salinity.
The accompanying Excel spreadsheet can be downloaded.
Further a powerpoint presentation is available. You may download it or otherwise see it on-line.
 

or a genuine interest?

 

(C) Soil subjects

 
figure soil salinity
Looking over the shoulder of a farmer to his
salinized land with poor crop stand.

    17 - What are saline and salty soils, how do they originate, where are they found, what is their problem, how can the problem be solved?

      This is the paper on soil salinity
There is also a programmed self-help exercise on water and salt balances and leaching (reclamation) of a saline, salty and sodic, soil in Chacupe, Peru. Soil salinity, sodicity and alkalinity are analysed: Chacupe case study
A description of salinity development and control in a small coastal delta in Peru can be can be consulted here.
See also "Salinity control" in Wikipedia, or: WaterWiki.

 
18 - What are alkali, alkaline sodic and acid sulphate (sulfate) soils (cat clays), how do they originate, where are they found, what is their problem, how can the problem be solved?

      This is a paper on soil alkalinity, sodicity and acidity
A report on acid sulfate soils in the island of Pulau Petak near Bandjermasin, South Kalimantan, Indonesia, can be viewed on line.
Here is an article on acid sulfate soils in Guinea Bissau.
See also "Acid sulfate soils" in Wikipedia, and: Alkali soils in Wikipedia.
 
        19 - How can hydraulic conductivity (permeability) of the soil be measured and determined?

                          The measurement of hydraulic conductivity can be done in the laboratory or in the field, above or below the water table, using small scale (auger hole method, infiltration method) and large scale methods (e.g. using drainage data in existing experimental fields or in farm land).
A full description is given in the chapter "Measurement of hydraulic conductivity of soils".
See also the table of contents on line.
For well pumping tests to determine aquifer hydraulic properties see the WellDrain model (see the software page).

Questions are good, problems are not
 

(D) Statistical subjects

     
20 - What are the principles of cumulative frequency analysis and (discontinuous) probability distributions (used in the CumFreq program, see the software page), and what to do if there is a discontinuity? What is return period and how reliable is it?

     This is the lecture note on frequency analysis.
The table of contents can be inspected here.
     An article on the use of the binomial distribution for the determination of confidence intervals and belts for cumulative probability distributions can be downloaded from this place, while a summary of it can be found over here.

21 - What are the principles of linear regression analysis (used in the SegReg program (see the software page), ) and how are confidence statements and intervals determined. What is breakpoint (break point), threshold, and coefficient of explanation (determination)?

     This is the lecture note on regression analysis.
The table of contents can be seen on line.
One may also consult a paper on crop production and soil salinity: evaluation of field dat from India by segmented linear regression with breakpoint.
The construction of confidence intervals along the regression segments with breakpoint and of the breakpoint itself can be found this confidence paper.

22 - The SegReg model (see the software page ) provides confidence intervals of the break-point in segmented linear regression analysis to test its statistical significance.
Is it necessary to additionally perform an analysis of variance (ANOVA) and F-test and how is this done?

     This is the Anova paper

23 - Is there a calculator available for the F-test described under the previous question?

     Yes, one may download the calculator from the F-test page here.

24 - Is there a calculator available for the t-test using Student's t-distribution in determining the confidence intervals in (segmented) linear regression anlysis?

     Yes, one may download the calculator from the t-test page.

25 - Is there also a calculator available for the cumulative frequency in the normal distribution?

     Yes, one may download the calculator from normal distribution page.

26 - Is there a technique to compare averages of different data series and to determine the significance of the difference between them.

     For this one uses Student's t-test. See the t-test page from where a t-test calcuator can be downloaded.
 

or a genuine interest?

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    For more papers proceed to the articles page, the published reports & cases page, the particular reports page or else return to the home page.                                                                                                                                                                        
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