FAQ's   Frequently asked questions   and   answers   on land and water magement

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 33 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
    A1 - What is waterlogging, how does it influence soil salinity and what is its relation with irrigation? (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 shallow water table inside the soil (at a depth of say less than 0.5 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).

A2 - What is the tolerance or senstivity of crops to shallow watertables?

    Field measurements of crop yield and depth to water table are scarce and it is therefore not easy to present crtiical watertable depths for various crops. Data found in literature for wheat, banana, sugarcane and cotton have been analysed on the water tolerance page, giving also a classification on tolerance/sensitivity that may give a help with the formulation of a drainage criterion for these crops.

A3 - What kind of drainage systems are applicable under what conditions? What is their relation to irrigation and soil salinity?

    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.
A4 - How can I calculate drainage discharge needs and irrigation/leaching requirements for salinity control in waterlogged land 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.

A5 - 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

A6 - 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).

A7 - Drain spacing equations for horizontal tile/pipe drains are well known. Can subsurface drainage also be done by (tube)wells and, if so, 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).

A8 - How do the required drain and well spacings under the same conditions compare?

    An example of comparison drain and well spacings for horizontal resp. vertical drainage can be found in the spacings page. The well spacings are much wider.

A9 - 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.
Examples of crop sensitivity to shallow water tables that has to be taken into account when formulating drainage criteria can be seen on the crops and water table page.
See also Wikipedia: Drainage research or: Watertable control.

A10 - What are the effects of drainage on agriculture?

    Possibly you may find the answer in this book chapter entitled: "The study of effects of drainage on agriculture".
In addition, an article entitled: "Agricultural land drainage: a wider application through caution and restraint" may be helpful in this respect. It can be seen at: this place

A11 - What are the design criteria for subsurface drainage systems?

    You may find the answer in this journal article entitled: "Agricultural Criteria for SubSurface Drainage: a Systems Analysis", which was published in "Agricultural Water Management", Vol. 14 (1988), p. 79-90.

A12 - How can surface drainage (runoff) be calculated from rainfall without using the complicated method with an instantaneous unit hydrograph (IUH) and convolution?

    On this website there is a method that uses a model of a non-linear reservoir for rainfall-runoff relations, and the corresponding software can be found on the RainOff page.
In addition there exists the method of a cascade of linear reservoirs, all having the same raction factor, as elaborated by Nash. The software is to be found on the NashMod page.
          Further, on request, one can obtain software of a computer program for the method using two linear resrvoirs in series with different reaction factors (response functions).

A13 - What is the influence of the soil depth on the behaviour of the water table in time?

    The determination of the influence of the soil depth is complicated due to the need to calaculate the equivalent depth. The following paper on the impermeable layer may be helpful to solve this problem.

A14 - What envelope materials (filters, wraps) are used for pipe drains during their installation?

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

A15 - 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
B1 - 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.

B2 - What about water management, irrigation, drainage 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.

B3 - Is irrigation high up in the mountains feasible?

      This is a paper discussing the importance of irrigation at high altidues in the Andean mountains.

B4 - 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. Spate irrigation is done by qanats or karezes. It is also called flood recession cropping.

B5 - 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.

B6 - How to determine the irrigation efficiency or sufficiency in a drainage project?

      This question is dealt with in the article on the DrainApp software.
The user interface of this software is to be found on its manual.

B7 - 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.

B8 - 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

(C) Soil subjects
figure soil salinity
Looking over the shoulder of a farmer to his
salinized land with poor crop stand.

figure soil salinity
Rice polders along a sea arm in Guinea Bissau amidst
the mangroves. There are acid sulphate soils.
    C1 - What are saline and salty soils, how do they originate, where are they found, what is their problem, how can the problem be solved? What is their relation to irrigation, waterlogging and drainage?
      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 here: Chacupe case study
     An article on the Chacupe case has appeared in the International Journal of Environmental Science, entitled "Reclamation of a saline vertisol by irrigated rice cropping, interpretation of data with a salt leaching model", see this website.
     In addition, 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

C2 - What is salt tolerance of crops and are there any determinations of crop response to soil salinity under farming conditions?
      In this tolerance paper examples are given of determination of salt tolerance based on measurements in farmers' fields on the relation between crop production and soil salinity in 3 subtropical countries.
See also the article that appeared in the International Journal of Agricultural Science, entitled: Crop Tolerance to Soil Salinity, Statistical Analysis of Data Measured in Farm Lands.

C3 - 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.
Alkalinity is strongly associated with the presence of soda (Na2C03, sodium carbonate) in the soil, ground or irrigation water.
A report on acid sulfate soils in the island of Pulau Petak near Bandjermasin, South Kalimantan, Indonesia, can be viewed on line.
The formation of acid soils occurs frequently in mangrove forest areas and is related to the presence of pyrite (Fe2S, iron sulfide). Here is an article on acid sulfate soils in Guinea Bissau.
See also "Acid sulfate soils" in Wikipedia, and: Alkali soils in Wikipedia.
Questions are good,
              problems are not

(D) Statistical subjects
D1 - What are the principles of cumulative frequency analysis and (discontinuous, composite) 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.
     An article using composite (discontinuous) probability distribtions has appeared in the International Journal of Mathematical Methods, entitled "Siftware for generalized and composite probability distributions, see this website.

D2 - 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.

D3 - The SegReg model (see the software page ) provides confidence intervals of the break-point (BP) in segmented linear regression analysis to test its statistical significance. How is that done? Is it necessary to additionally perform an analysis of variance (ANOVA) and F-test and how is this done?
     The calculation of the confidence interval of BP is done using the laws of propagation of errors as explained in the BP article.
This is the Anova paper. Sofware for the F-tets can be found on the

partial regression wheat and salt
The wheat crop tolerates soil salinity (measured in electric conductivity, ECe) up to a level of 7.1 dS/m, beyond which the yield declines.
D4 - Is there a away to determine the longest horizontal stretch while fitting a relation with a horizontal and sloping part to a number of data? This stretch is needed to find out the range over which there is no effect of the influential variable on the goal variable.
     Recently I developed a method called "partial regression" eactly for this purpose. It can be found on the PartReg page. It can be used as a method to find the salt tolerance of crops as well as the sensitivity response to high watertables. See also the applications in this: tolerance paper and the: water table paper.

D5 - 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.

D6 - Is there also a calculator available for the cumulative frequency in the normal distribution?
     Yes, one may download the calculator from normal distribution page.

D7 - 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.

D8 - Is there a technique to to fit S-curves to or third degree (cubic) polynomials to crop yield data to model the response to changes in growth factors.
     For this one can use the SegRegA model as explained on the S-curve page respectively the cubic regression page.

D9 - How are plotting positions used in the determination of a probability distribution for a series of data?
     On this subject an article was written entitled: "How to derive a robability distribution from a data set using the simple method of plotting positions and the free CumFreq model" was entered, see plotting.

or a genuine

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