Harvested economic yield in metric tonnes per hectare per harvested area reported at standard moisture content appropriate for each commodity. 1 tonne is 1000 kg.

In irrigated systems, yield potential or potential yield (Yp) is the yield of a crop cultivar when grown with water and nutrients non-limiting and biotic stress effectively controlled (Evans, 1993; Van Ittersum and Rabbinge, 1997). Therefore, crop growth is determined by solar radiation, temperature, atmospheric CO₂ concentration, and genetic characteristics. Yp is location specific because of the climate, but, in theory, not dependent on soil characteristics. Yp is used as benchmark for estimating yield gaps only for fully irrigated crops. Yp is not shown for rainfed crops, although Yp is used for estimating the water limitation index (see definition below).

For rainfed crops, water-limited yield potential (Yw) is the most relevant benchmark. Yw is defined similar as Yp, but crop growth is also limited by water supply, and hence influenced by soil type and field topography. Yw is used as benchmark for estimating yield gaps only for rainfed crops, hence, Yw is not shown for irrigated crops.

For irrigated crops that do not receive sufficient water to fully satisfy their requirements, irrigation amount and timing need also to be considered for simulating the partially-irrigated crop yield potential (Ypi). This term is equivalent to ‘deficit irrigation' yield potential used by others (English, 1990). Ypi is used as benchmark for estimating yield gaps in partially-irrigated irrigated crops.

Average yield (Ya) is defined as the average (of for instance the past 5 years for irrigated and 10 years for rainfed cropping systems) yield achieved by farmers in a given region under dominant management practices (sowing date, cultivar maturity, and plant density) and soil properties.

The yield gap (Yg) is the difference between Yp (irrigated crops), Yw (rainfed crops) or Ypi (partially- irrigated crops) and actual yield (Ya). Yg is based on Yp, Yw or Ypi simulated using optimal agronomic management as input (i.e. cultivar maturity, sowing date and planting density). In GYGA, optimum sowing dates, plant density, and cultivar maturity are based on dominant practices currently used by farmers. However, sowing dates, planting density, and cultivar maturity differed from actual practices when there is strong evidence to support using an optimum value, such as published field research that documents higher yields with sowing dates different from those currently used by farmers. In those cases, the optimum sowing date and cultivar maturity are determined within the time constraints of the dominant crop sequence (e.g., areas where more than one crop is grown on the same field each year).

Yp and Yw are defined by crop species, cultivar, climate, soil type (Yw, Ypi), and water supply (Yw, Ypi), and thus they are highly variable across and within regions. It is impossible for a large population of farmers to achieve the perfection in crop and soil management required to achieve Yp or Yw or Ypi, and it is not cost-effective to do so because yield response to applied inputs follow "diminishing returns" as average farm yields approach Yp or Yw or Ypi (Koning et al., 2008). Therefore, average farm yields often begin to plateau when they reach 75 to 85% of Yp or Yw or Ypi, and the exploitable yield gap (Yg-E) is defined as the difference between 80% of Yp or Yw or Ypi and current average farm yields (Cassman et al., 2003; Lobell et al., 2009).

Annual yield or yield gap is the total yield produced per hectare when more than one crop is planted and harvested on the same field within a 12-month period.

Water resources to support rainfed and irrigated agriculture also are under pressure, making the efficiency with which water is converted to food, water productivity (WP), another critical benchmark (Bessembinder et al., 2005; Passioura, 2006; Grassini et al., 2011) of food production and resource use efficiency. Water productivity is defined as the ratio between (grain) yield and seasonal water supply, which includes plant-available soil water at planting and minus available soil water at harvest and other unavoidable field-level water loses through deep percolation and surface runoff. WP is only calculated for rainfed crops. For more information see the methods page on calculation of water productivity.

The gap between the water-limited yield potential (Yw) or partially-irrigated yield potential (Ypi) and the yield potential (Yp), which assumes no limitations due to water supply) as a percentage of Yp, so (1 – Yw [or Ypi]) / Yp) × 100%. For a given location, the water limitation index is calculated for a single crop within the current cropping system and farm management regime. Water limitation index is only calculated for rainfed and partially irrigated crops.

Information that defines a cropping system consists of the number of crops on a given field per year (cropping intensity: single, double or triple cropping), including the accompanying cropping periods from sowing to maturity for each crop cycle and whether each crop is grown under rainfed or irrigated conditions.

For a specific crop, the cropping intensity is the number of times that crop is grown in one year on the same field. We distinguish single, double and triple cropping systems, with, respectively, cropping intensity 1, 2 and 3. In regions where farmers practice different cropping intensities, a weighted average of physical area in each system can give a cropping intensity that is not a whole number. For example, if in a climate zone (CZ), 75% of physical area is under a double-crop system and 25% under a single crop, the mean cropping intensity of that CZ is 1.75.

To minimize data collection and focus the yield gap analysis on areas of greatest relevance, we seek to identify regions with greatest contribution to national production totals for a specific crop and water regime. At issue is how to define the inference domain for a given Yp, Yw, and Yg estimate. To this end, we define an agro-climate zone (CZ) as a geographic region characterized by homogeneous climate conditions.

A region in which climate and root-zone soil water holding capacity are relatively uniform such that performance of crops and cropping systems, and associated crop and soil management options, are expected to be similar.