Grass-Cast Southwest: A seasonal rangeland productivity forecast for the southwestern United States

Authors: AGUILAR-CUBILLA, EMILIO - University Of Arizona; HARTMAN, MELANNIE - Colorado State University; PARTON, WILLIAM - Colorado State University; REED, SASHA - Us Geological Survey (USGS); Derner, Justin; SCHULTE, DARIN - Colorado State University; GORNISH, ELISE - University Of Arizona; MOORE, DAVID - University Of Arizona; Elias, Emile; Peck, Dannele; FUCHS, BRIAN - University Of Nebraska; SMITH, WILLIAM - University Of Arizona

Submitted to: Drylands
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2025
Publication Date: 11/28/2025
Citation: Aguilar-Cubilla, E., Hartman, M.D., Parton, W.J., Reed, S., Derner, J.D., Schulte, D.K., Gornish, E.S., Moore, D.J., Elias, E.H., Peck, D.E., Fuchs, B.A., Smith, W.K. 2025. Grass-Cast Southwest: A seasonal rangeland productivity forecast for the southwestern United States. Drylands. 2. Article e16. https://doi.org/10.1017/dry.2025.10013.
DOI: https://doi.org/10.1017/dry.2025.10013

Interpretive Summary: Ranchers and other land managers in the semi-arid rangelands of the Southwest US need improved forecasting tools for aboveground productivity to be used in decision-making for livestock production and other goals. United States Department Agriculture (USDA) developed a rangeland productivity forecast tool, Grass-Cast, for the Great Plains and this tool has recently expanded to the Southwest US. Grass-Cast Southwest provides short-term seasonal forecasts of rangeland productivity for Arizona and New Mexico for the spring (April to May) and summer (June to September) growing seasons. Three years (2020-2022), encompassing wet and dry growing seasons, were used to assess the effectiveness of Grass-Cast Southwest in forecasting rangeland productivity. Across years, Grass-Cast Southwest forecasts in early April were relatively close to the final observation-based production values. In contrast, the first summer Grass-Cast Southwest forecasts, produced in June, were not consistent in predictive ability to the final observation-based production estimates in late August, due to the variable impacts of the monsoonal precipitation amounts. Addition of El Niño Southern Oscillation (ENSO) indices improved Grass-Cast Southwest performance in both the spring and summer growing seasons. Our assessment of Grass-Cast Southwest across growing seasons with high interannual variability identifies key strengths and challenges associated with rangeland productivity forecasts for the Southwest geographic region.

Technical Abstract: Here, we present a first assessment of the US Department of Agriculture’s (USDA) “Grass-Cast Southwest,” which is a forecasting tool for rangeland aboveground net primary productivity (ANPP) for the southwest region of the United States. Our results show that ANPP forecasts in early April were relatively close to the observation-based ANPP estimates in late May for all years evaluated (R = 0.6–0.9). The relatively high predictability of spring rangeland productivity in this region is likely because it is strongly driven by antecedent winter/early spring precipitation. Conversely, the first summer forecasts produced in June did not consistently predict the final observation-based ANPP estimates in late August (R =
0.5–0.7), likely because summer rangeland productivity in this region is highly dependent on variable, less predictable precipitation from the North American Monsoon (NAM). Antecedent El Niño Southern Oscillation (ENSO) indices could be used to improve Grass-Cast Southwest performance in both the spring and summer. The ENSOJFM (January–March) index was significantly positively correlated with rangeland productivity during the spring season, whereas ENSOMAM (March–May) was significantly negatively correlated with rangeland productivity during the summer season.