Methods used to estimate yield gaps in the Yield Potential and Yield Gap Atlas (YPYGA) were developed to improve:

Previous yield gap analyses are too coarse or too empirical to serve these purposes, and they lack transparency with respect to methods, sources of data, and underpinning assumptions, which makes it difficult to validate estimates or improve upon them [3].

To address these deficiencies, YPYGA utilizes methods that are transparent, reproducible, and based on the best available science. Yield gaps (Yg)

are estimated by the difference between yield potential (Yp)

without limitations due to water or other abiotic and biotic stresses (the most relevant benchmark for irrigated systems), or water-limited yield potential (Yw)

as the benchmark for rainfed systems, and actual crop yields (Ya)

. Water productivity (WP)

is estimated as the ratio between Yw and crop water availability (potential WP) or between Ya and crop water availability (actual WP). Crop water availability is defined as the amount of water supply available during the crop growing season (from available soil water at sowing, precipitation, and also irrigation in irrigated systems) after discounting unavoidable water losses through surface runoff, deep drainage and the residual available soil water left in the soil profile at crop physiological maturity.

Eco-physiological crop simulation models are used to estimate Yp, Yw, and crop water availability to account for the influence of climate, soil-type and length of growing season as determined by climate, cultivar and dominant cropping systems at each location. In many parts of the world with sub-tropical and tropical climates, farmers grow two or three crops per year on the same field. In these regions farmers strive to optimize productivity of the entire cropping system rather than yield of a single crop to maximize returns to labor, land and time. Therefore, estimates of Yp, Yw, and WP for a given crop in those systems must account for actual length of growing season as constrained by the cropping system.

Fig. 1. Different production levels as determined by growth defining, limiting and reducing factors [2][3][4][5]). For irrigated systems and rainfed systems, respectively, potential yield (Yp) and water-limited yield (Yw) are the relevant benchmarks for yield gap analysis.

The "bottom-up" approach of the YPYGA protocol allows use of actual location-specific data on current cropping systems, soil type, and long-term weather data from selected reference weather stations (RWS) so that results can be validated and improved upon as quality of such data improves and becomes available in major crop-producing regions worldwide. Where such data are of poor quality or not available, YPYGA utilizes the best existing data, or proxy data, with the goal of improving upon these data over time in a "wiki" concept. In all cases, sources of the underpinning data are specified and they are publicly accessible on YPYGA website (to the extent allowed by each source) for users who wish to perform their own yield gap assessment using other methods or to extend the analysis to include other crops and production scenarios. Use of robust crop simulation models to estimate Yp, Yw, and WP also allows assessment of yield stability and risk due to high or low temperatures and variability in rainfall at a given location.

Up-scaling location-specific estimates of Yp, Yw, Yg, and WP to regional and national levels is achieved by ensuring all major climate zones

(CZs - as defined by van Wart et al., 2013), soil types, and cropping systems are covered by the location-specific analyses at RWS, and by aggregating location-specific estimates to larger spatial scales by weighting for harvested crop area under each regime. These up-scaling methods are also transparent and reproducible, and they are applied consistently across crops and countries.