Field Application Efficiency

Field Application Ratio

Some Background

The field application ratio Ra (efficiency equals 100 x Ra) was developed some 50 years ago as an performance indicator downstream of the conveyance- and distribution ratios (Bos and Nugteren 1974). At that time, they formulated the following four serious drawbacks due to the lack of knowledge on water utilization efficiencies:

  1. In the planning and design of irrigation systems a large safety margin is applied, as a consequence of which irrigation facilities like canals, structures, and reservoirs are constructed with capacities that are too large.

  2. Investments are considerably higher than otherwise be necessary.

  3. The limited water resources are not optimally distributed and used, as a result of which much water goes to waste and less land can be irrigated.

  4. Last, but not least, the low overall irrigation efficiency creates harmful side effects such as rising groundwater tables and soil salinization. To control the groundwater table a costly subsurface drainage system may be necessary and this will seriously affect the economy of the project.

By now, the focus is on part of the third drawback while the others are neglected. By neglecting the water balance aspects of irrigation, the efficiency of water use, particularly at field level, is often misused. The most common pitfalls are listed below. To avoid these pitfall you may download a extract of Chapter 5 of the above manual.

Common Pitfalls:

  • Misuse and mixed use of the terminology on “used water” and “consumed (evapo-transpirated) water”. Used water that is not consumed can be reused if its quality remains acceptable.

  • Assume that the non-consumed part of the used water is lost. In reality this “lost” water either recharges the aquifer or becomes surface drainage. This water, however, becomes available for further use. Always think water balance!

  • Assume that the irrigation efficiency should approach 100%. This implies, however, that all salts in the applied water will be stored in the root zone until leached by infiltrated (off-season) rain. With insufficient rain agriculture will be unsustainable.

  • Believe that so-called “modern techniques” like sprinkler and drip are more efficient than the properly leveled surface irrigation methods. This believe is greatly stimulated by advertising. In reality, laser leveled surface irrigation often is superior to the above “modern techniques”.

  • Believe that raising the efficiency of water use will “create more water”. In reality it means that ETa will increase, resulting to less recharge and drainage. Considering the water balance, however, a close study of the field water balance shows that this increase of ETa results to a reduction of available water to the next downstream user.