Good, relatively inexpensive, and easy to operate sensors are the Decagon EC-5 and 10 HS as well as the Acclimation TDT probe.

http://www.decagon.com/products/sensors/soil-moisture-sensors/

http://acclima.com/wd/index.php?option=com_content&view=article&id=11&Itemid=9

See also: VAZ, C.M.P, S.B. JONES, M.S. MEDING, and M. TULLER, 2013. Evaluation of Standard Calibration Functions for Eight Electromagnetic Soil Moisture Sensors. Vadose Zone J., 12(2), doi:10.2136/vzj2012.0160.

I have had experience on using the Sentek soil moisture sensors (http://www.sentek.com.au/home/default.asp). Specifically, the Enviroscan probes (http://www.sentek.com.au/products/monitoring.asp). They work fine and the cost is not so high. I know these probes are widely used in Australia for agricultural purposes.

Thanks a lot, Drs. Tuller and Brocca. I'd appreciated your attention so much. Your answers are precisely on my question - measurements of soil moisture. And, the sensors you have described are really suitable for calculating the right amount of water into the soil. Or else, they permit to calculate precisely the "correct amount of water" for each irrigation. Maybe, multiple sensors (such as described by Dr. Brocca) can be not so cheap or simple for small farmers. Even so, they are quite suitable for many purposes.

But, my question has another sense, because we have to consider the variability of soils. And, we know that plants respond to the soil water potential, rather than to the soil water content. This means that different soils may offer to plants different water conditions, even with similar water contents. Besides, in a same cropping area we can have a high variability of soil-water relations. And, the "right moment to irrigate" depends on this aspect. We know that tensiometers can be an alternative for that, but it seems that they remain as good sensors only for research purposes. At farm level, they are not so convenient for continuing operations. Is there any alternative at the moment?

Dear Homero,

Dealing with spatial variability is still a big challenge especially when you are looking for inexpensive sensors for small farms that are easy to operate and maintain. Besides their limited measurement range, tensiometers require frequent maintenance.

Decagon offers the relatively inexpensive and easy to use MPS-2 water potential sensor that can be used to trigger and cease irrigation based on matric potential measurements (http://www.decagon.com/products/sensors/water-potential-sensors/mps-2-dielectric-water-potential/), but you are still left with the problem of finding the right strategic locations for sensor placement to capture spatial heterogeneity.

A good method for rapid mapping of spatial textural contrasts is Electromagnetic Induction (EMI). Please see an example for mine tailings in: NEARING, G.S., M. TULLER, S.B. JONES, R. HEINSE, and M.S. MEDING, 2013. Electromagnetic Induction for Mapping Textural Contrasts of Mine Tailing Deposits. Journal of Applied Geophysics, Vol. 89:11-20, doi:10.1016/j.jappgeo.2012.11.005. This of course requires specialized equipment and needs to be done by an experienced user once. Based on the obtained resistivity map you can decide on the best locations for your sensors for irrigation scheduling. I am not really sure if this is feasible for a small farm.

There are also farmers that use tractor-mounted geo-referenced TDR systems to generate moisture maps for irrigation scheduling by grid-like probing large fields.

Best regards,

Markus

Dear Markus,

Thank you very much. Besides of the wide range of operation (-10 to -500 kPa), the MPS-2 sensor may have some advantages in humid regions, like here in Southern Brazil, where the water conditions (rain and evaporative demand) are very irregular in summer seasons. We don't have conditions do establish a regular irrigation schedule. So, decisions on irrigation management are more complicate than in dry regions.

Best regards. Homero.

.

Dear Traore,

actually, the cost depends on many things.

We have usually installed Enviroscan probes with three sensors at 10, 20 and 40 cm depth. I'm not really sure of the cost. But, without a connection via radio (or via GPRS), for one probe with three sensors it should be around 1000 euro. However, I suggest you contacting Sentek for more precise information.

Cheers,

L

As regard soil moisture (but not soil water potential) spatial-temporal variability, we carried out several soil moisture campaigns, in different areas and at different spatial scales, for studying this issue.

We found, in accordance with many other studies, that few sensors (2-3) are able to provide an useful information for monitoring soil moisture temporal variability over a large area (e.g. an agricultural field) with a good accuracy.

You can find our publications on this topic on Research Gate. However, if you need more info just let me know.

L

Hi Homero, Why not use indirect methods such as TVDI?

Dear Miguel,

Please, give me some details about those indirect methods, in particular TVDI.

Thanks in advance.

Regards. Homero Bergamaschi.

I hope I can help a little.

There are several types of indexes used to determine the soil moisture indirectly. some index i know are: CWSI (Crop Water Stress Index), WDI (Water Deficit Index) and TVDI (Temperature-Vegetation Dryness Index).

I've only read about TVDI.

The TVDI is an estimate based between the NDVI and surface temperature (LST) proposed by Sandholt (2002). The advantage of this index is to be calculated directly from satellite data, without the need for auxiliary information. The disadvantage is that it is not well defined in large areas, so validating TVDI observation becomes difficult.

The TVDI is inversely correlated with soil moisture.

The LST and NDVI respond to a linear function, in dry conditions, the vegetation index is increased by increasing the density and vigor of vegetation, the LST decreases in vegetation exposed to sunlight compared to bare soil, creating a slope in relation LST / NDVI. In conditions of high humidity surface, LST is the low and similar for both bare soil to vegetation, giving a planar relationship between LST and NVDI.

TVDI value is between 0 and 1. 0 represents a saturation of soil moisture while 1 does not contain soil moisture.

Yang (2008), changes TVDI proposal Sandholt . changing NVDI by EVI, concluding that in most of the case it is better to use EVI.

In Sandholt explains the theoretical support for this method.

Sandholt, I., Rasmussen, K., y Andersen, J. (2002). A simple interpretation of the

surface temperature/vegetation index space for assessment of surface moisture status. Remote sensing of enviroment, (79):213–224.

Yang, X., Wu, J. J., Shi, P. J., y Yan, F. (2008). Modified triangle mehtod to estimate

soil moisture status with moderate resolution imaging spectroradiometer (MODIS)

products. En The international archives of the photogrammetry, remote sensing and

spatial information sciences, volumen XXXVII, pg 555–560.

Dear Bergamaschi

I have used "PR2 - Profile Probe- Delta-T Devices Co. " for measuring the soil moisture in a sunflower farm (as my thesis).

It works well!

You can find more details here:

http://www.delta-t.co.uk/product-display.asp?id=939&div=Soil%20Science&cat=

Dear Mina,

Thank you, very much.

We need good alternatives, and your suggestion is quite important.

Best regards.

Homero Bergamaschi.

One of the best reports dealing with soil water measurements, in the IAEA report/course called "Field Estimation of Soil Water Content A Practical Guide to Methods, Instrumentation and Sensor Technology" (attached)

In this report you will find every thing about almost all the devices and methods used to measure S-W content.

In brief, you have to specify three things

1- your soil type (texture) as some methods are suitable only for some textures and soils.

2- the frequency you want to measure at (weekly, daily, hourly, or even minutely)

3- the precision level of measurements (for irrigation control or for verifying a model)

if you want maximum precision for all textures but with low frequency, then the gravimetric method is unbeatable, if you want high frequency with low precision then use capacitance probes like Enviroscan.

Dear Mohammad Elnesr,

Thank you very much for the contact.

I could download very easily the text from IAEA (2008). It seems to be really useful in describing the different methods, instruments, and sensors for measuring and estimating the soil water content. I'm sure that many people will be interested on that practical guide.

Your contribution is very important.

Best regards,

Homero Bergamaschi

Thanks Homero, I hope everybody benefit from it.

Dear Rui Machado,

Is it possible to show some details about the "watermaks"?

Um grande abraço e obrigado pela sugestão.

Homero Bergamaschi.

Dear Miguel Armando López Beltrán,

We have some colleagues working on remote sensing applied to agricultural purposes, in particular with NDVI and similar parameters. I sent to them your suggestions. Thanks for them.

Un gran saludo.

Homero Bergamaschi

Dear Rui Machado,

Thank you for the paper. Details on monitoring the soil-water potential are interesting.

Regards.

Homero Bergamaschi

Dear Rob Blakey,

You're right. Tensiometers are really trustworthy for monitoring the soil water potential up to around -0.07 or -0.08 Mpa. We have used mercury tensiometers in our experiments for a long time. And, data used to be quite precise for many purposes, in particular when establishing water relations in the soil-plant-atmosphere system. The physical principle of water potentials has several advantages for deep evaluations into the entire system.

However, in general, farmer and even our colleagues of extension services don't appreciate using tensiometers, irrespectively of models and operation systems. That was (more or less) my statement, at beginning of the question.

On the other hand, there are some alternatives for monitoring the water moisture based on matric potential measurements, as suggested Markus Tuller.

Well, it appears that instruments have to be easier (and cheaper) to be largely used at farm level, and people must be skilled to be able for using them.

Best regards.

Homero.

Homero,

I use Gro Point Soil Moisture Meters Environmental Services Inc.100-4243 Glanford Ave., Victoria, BC., Canada, Tel 250 479 6588. I set them at various locations (some areas of the field will retain moisture longer due to soil type) and depths (6" and 12") to monitor the impact of various application rates of irrigation water. The system works great once you establish what application rates you can apply, You will need to record data to fine tune YOUR individual application system. You should take into account the Relative Humidity at 2:00 PM, amount of rainfall in the previous 24 hour period, wind speed and crop development stage. All these factors will impact the evapo/transpiration rate.

Jon Cloud

Field servers are getting popular these days....

Amor

I use the meters not just for raising crops. But rather, I irrigate various crops using treated wastewater that has been treated in two ponds, one with obligated aerobes and one with algae. I drive the BOD to 2 and irrigate. This returns N and P to the fields were it belongs, uses the water to grow great crops, and the excess returns to the aquifer or evapo/transpiration moisture to the atmosphere where it usually precipitates out within 200 Km. Twenty one million gallons so far this year. Awesome results. The soil moisture levels I operate at are not anything close to what normal irrigation runs at. I am trying to optimize uptake and transpiration; while normal crop irrigation operates slightly above plant stress moisture levels. Same process, different motivations and requirements.

Jon Cloud

We did a comparison of different sensors and I am attaching the poster or the results.

Here is a paper comparing the performance of five different sensors.

Dear Ramanathan Sri Ranjan,

Thanks for your contribution. It was possible to read the attached "poster" (15 pages) and the results are very clear and interesting, considering both the technological and methodological aspects.

On the other hand, if trying to extend your conclusions to a farm level (on field conditions): How would you compare the performance of the sensors? Or else, how would you say about the convenience in using each one, in a monitoring routine?

Best regards. Homero Bergamaschi.

Soil variability in the field affects the performance of the probes. The capacitance based probes are affected by air-gaps adjacent to the probes. Therefore, careful installation is important. I am attaching another paper containing results from a study of the same probes in potato fields.

I am attaching the poster which I mentioned in my previous message but accidentally got omitted.

Here is the paper on the field study. I am new to this site and clicked the wrong button before I could attach the paper.

Dear Ramanathan Sri Ranjan,

Both articles are very interesting. Thanks for attaching them to everybody.

Best wishes.

Homero Bergamaschi.

Thanks ramanathan, interesting articles.

Since none of the soil moisture meters in the study," Laboratory Evaluation of Soil Moisture Sensors" was always accurate through the entire range of soil levels, It must be clear the brand of instrument maybe less important than the powers of observation by the operator in monitoring and recording stress in the plants. Rotating irrigation pivots deposit specified mm of water on a variety of soils in a single rotation. It would seem to me to be prudent to monitor the moisture levels of each soil type throughout the year against a final yield comparison. Science is not an exact science and neither is farming. The moisture meters are tools not decision makers.

Jon

Any capillary pressure based sensors i.e. WaterMark sensors, tensiometers, report the soil moisture indirectly as capillary pressure which is independent of the soil type. Ultimately, it is the capillary pressure of the soil that the plant has to respond. However, different soil types will hold different quantities of water at the same capillary pressure making the irrigation capacity highly dependent on soil type. Accurate estimation of the depth of irrigation for a given soil type is only possible if we have a good soil moisture characteristic curve for the given soil type. In Manitoba, Canada, the soil physical properties of the entire agricultural soils have been characterized and embedded as layers in a GIS map called "Agri-Maps" which is freely accessible on the web. It makes it easier for farmers to locate their field and get the soil characteristics from Agri-Maps. Now that variable application technology is gaining ground, the farmers can program their center pivots to do site-specific application.

Ranjan

Ranjan,

We program our pivots presently. Also I would add slope as well as soil type will provide a large variable. Additionally, I bury meters at 6" and 12" depths to monitor the impact of rain and irrigation. Once of the major difference between most farmers and us is the quantity of water being applied. Most farmers strive to reduce costs through minimum applications. I am just the opposite. I want to optimize the transpiration rate to maximize water quantitative out put. The more water I can evacuate the better. So I hold my moisture levels well below saturation but at much higher levels than potato growers. The growth stage is the largest factor for irrigated water. I have made applications during various growth stages of corn with the transpiration being so high the meters registered no change in soil moisture. So growth stage. rules.

Jon

Dr. Ramanathan Sri Ranjan

I think you're right.

Plants respond to the soil-water matric potential, which you called capillary pressure. And, depending on the porous geometry, the soil will hold different quantities of water at the same water potential, making the irrigation capacity highly dependent on soil physical conditions. Therefore, it's necessary a good soil moisture characteristic curve for each given soil type (for converting the soil water content), in particular for an accurate estimation of the depth of irrigation. This seems to be very clear.

On the other hand, besides of "How much water you have to apply?" there is always another question: "When the irrigation must be applied to plants?" Sometimes, this aspect is the most important than the first one, in particular during critical stages of crops with a high evaporative demand. In other words, since water potential based sensors tend to reproduce the plant sensibility to the water conditions, independent of the soil physical conditions, they have some advantages over other types.

Am I wrong?

Best regards.

Homero.

Homero,

You are right in stating that the water potential based sensors are better at predicting the "trigger point" for irrigation. In fact, tensiometers come equipped with set point capability that can be set to "close a circuit" which in turn can telemetrically trigger a pump or alert an operator. We setup such a system to trigger batttery-powered flashing lights mounted above the crop canopy so the farmer could easily see which fields have reached the set point the previous night!

Ranjan

Dr. Ranjan,

We know that regular tensiometers have a limited range of functioning. However, this is not a real problem if the "trigger point" for irrigation is located above that limit (e.g. around - 0,07 MPa). In general, this is the case of sensible crops (vegetables, flowers ...) or even in critical stages of grain crops under high evaporative demand.

Besides, as you described, there are some modern devices that can improve the practical use of tensiometers (or other water potential based sensors) for indicating the "trigger point" for irrigation.

Have you any catalogue (or something like this) describing those equipped tensiometers, according to your description?

Thanks for your contribution. Homero.

Dear Homero,

Here is a link to the tensiometer that could be used to trigger a circuit:

http://www.irrometer.com/pdf/Irrometers/117_Automation_Output_Options_WEB.pdf

You can manipulate the out put to suit youee needs.

Ranjan

Good idea ramanathan

Dr. Ranjan,

Thanks a lot. Your contribution is really important.

Best regards.

Homero.

Don't use watermark sensors

They have upto 200 percent error at potentials near saturation

Check out the decagon devices mp4 (?) , however the best approach for measuring matric potential is heat pulse devices ( Campbell Scientific) but they require quite a bit of power. Back to your original question, at the whole farm scale there is only one approach which is the use of cosmos sensors which measure returning cosmically rays, the sensor works over a 200+ meter radius, and senses to about 10-30 cm depth. They are still under development but are commercially available .

I suggest having a look to this very recent paper:

"State of the Art in Large-Scale Soil Moisture Monitoring"

http://xenon.colorado.edu/portal/publications/Ochsner_etal_2013.pdf

Dear Homero

tensiometer is using for measurement of soil suction in the field. by knowing soil suction you can manage the irrigation time.

Good I could see several answers. Apart from the insitu measurements, microwave remote sensors are capable of measuring soil moisture. Microwave remote sensing provides a unique capability for direct observation of soil moisture.. Ofcourse, accurate soil moisture estimates are limited to regions that have either bare soil or low to moderate amounts of vegetation cover. . The spatial resolutions of passive microwave soil moisture sensors currently considered for space operation are in the range 10–20 km. The most useful frequency range for soil moisture sensing is 1 to 5 GHz. A good amount of research is being carried out to study the effects on microwave observations of within- the large pixel of spatial heterogeneity of vegetation cover and subsurface soil characteristics.,

Dear Lyyanki Muralikrishna thanks for your contribution.

Of course, remote sensors must be considered as a suitable alternative in many situations, depending on the resolution and precision required. In this aspect, I'd appreciate to know something more, if possible.

Best regards.