lecture slides

Most likely, you are familiar with Crop Land Productivity, the amount of biomass produced per area, also known as yield. 

Crop Water Productivity (CWP) is defined similarly as the amount of harvested biomass produced per area (yield) divided by the amount of water used to produce this biomass (evapotranspiration).

Let's discuss two components, yield and evapotranspiration, separately.

Yield: numerator

Agricultural crop yield is the total weight of harvestable product (biomass) produced per area. Fresh weight of potato tubers divided by the area of the field they were grown, fresh weight of grapes divided by an orchard area etc. 

Topic 2: GPP to yield discusses how to assess yield with remote sensing

Evapotranspiration: denominator

Biomass is produced by plants that need water to grow. Water is taken up from soil by plant roots, transported within the plant vascular system (xylem) and released through leaves and green stems. Transpiration is the process of water loss through leaves. Knowing the transpiration rate (mm per day) and the duration (growing season), we can compute the amount of water taken up by plants. For plant physiologists, transpiration is the water used to produce yield and net CWP, computed as yield / transpiration, is the CWP metric.

However, not all water supplied to a crop by irrigation or rain is transpired, some water is lost without being used by plants because soil loses water through evaporation. For a farmer, irrigation manager and economist, both evaporation and transpiration (i.e. evapotranspiration) have to be accounted for while computing CWP.

 

Advanced clarification:

There is also a broader definition of CWP that is based not on the harvestable biomass (yield as we defined it) but on total (fresh or dry, aboveground or aboveground + belowground) biomass. With the help of Topic 2: GPP to yield you can find the conversion factors between different biomass components.

Reading task:

Please, go through sub-sections 2.1.1 Water Productivity (raw pdf pp 22-24, document pp 10-12) and 2.1.2 Crop Water Productivity (raw pdf pp 24-26, document pp 12-14) of the WaPOR v2 methodology.

Answer the following questions:

  • What is the difference between Water Productivity (2.1.1) and Crop Water Productivity (2.1.2) that the manual wants to highlight?
  • Are those definitions in line with what you have heard in the lecture?

Last modified: Monday, 1 December 2025, 10:42 AM