Transpiration efficiency denotes the mass of dry matter produced per unit of water transpired during the growth period. Transpiration and crop yield are linearly and strongly correlated. The transpiration rate of a given crop is influenced by many factors, but reducing soil evaporation can make more water available for plant transpiration increasing yield and water productivity. .

Crop-coefficient is the ratio of actual crop ET to reference crop ET at a specific time (ETc/ETo). While ETo accounts for variations in weather, and is used as an indicator of atmospheric demand for water, Kc values account for the difference between ETo and ETc and link them. Each crop has a set of specific crop coefficients to predict water use rates at different growth stages

Crop co-efficient depends on the type of crop, growth stage, crop height, soil cover, and soil moisture. Among these factors, the ground cover and soil moisture can be manipulated and kept at almost uniform for the crops grown. In crop coefficient(Kc), the influence both evaporation and transpiration are included. If you are able to reduce evaporation, you can set apart more water for transpiration resulting in a better yield.

The ETc from crops grown under management and environmental conditions that differ from the standard conditions is designated as the crop evapotranspiration under non-standard conditions (ETc adj). This deviation from actual ETc is due to non-optimal conditions such as soil salinity, low soil fertility, water shortage, waterlogging, or because of pests and diseases. These can cause non-optimal scanty plant growth, low plant density, eventually reducing the ET rate below ETc. The crop ET under non-standard conditions can be estimated using a water stress coefficient, Ks and by adjusting the crop coefficient (Kc) for all kinds of other stresses and environmental constraints on crop ET. Please note that water stress coefficient (Ks) is less than 1 for soil water limiting conditions ( for maximum stress, Ks =0) and when there is no water stress, Ks = 1. Using the values of ETo, Kc, and Ks we can calculate ETc adj = Ks Kc ETo

Soil water stress reduces both Kc and ETc. Definitely, an increase in stress indicates reduction in yield.

Tewodros,

You can find good tips for your question in FAO Irrigation and Drainage Paper, No. 56, chapter 6.

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