ALSIM 1 (LEVEL 2) is a dynamic computer simulation model of alfalfa (Medicago sativa L.) growth and management written in the computer simulation language CSMP III. The model was developed primarily for studies of the management of defoliating pests of the alfalfa ecosystem. It is a refinement of ALSIM 1 (LEVEL 1), and for most applications it should be used in preference to LEVEL 1. The main improvement in LEVEL 2 is a soil water budget allowing for simulation of the effects of limiting soil water supply on alfalfa growth. Under most conditions, it will more accurately predict seasonal yield distributions than will LEVEL 1. Over-winter use of stored food reserves has also been added to LEVEL 2. Input data needed for LEVEL 2 are (a) yields of leaves, stems, basal buds, and total non-structural carbohydrate reserves (TNC) at the start of simulation; (b) soil water holding capacity of the root zone; (c) dates of harvest; (d) latitude of the study location; and (e) daily weather data for solar radiation, mean air temperature, and precipitation. The model predicts the yield of alfalfa hay and the growth curves for leaves, stems, basal buds, and TNC with simulated time steps of one day. It also simulates the supply of available water in the root zone on a daily basis. The model assumes largely pure stands of alfalfa, and it does not include growth limitations caused by excess soil water content. Neither does the model predict root growth and yield, assuming a root system sufficient to extract the available water. A discussion of model development, performance, and use is included in this report.