The GLEAMS model (Groundwater Loading Effects of Agricultural Management Systems) is used to illustrate model application in evaluating potential pesticide runoff of two similar pesticides from one soil. This limited application was chosen for simplicity in illustrating relationships between annual means and single events. When using annual totals of simulated pesticide runoff for comparing two pesticides or assessing risks, long-term 50-yr simulations are preferable to short 10-yr simulations. When short-term simulations are performed, care must be exercised in selecting representative climatic periods. For short half-life pesticides, as demonstrated in this study, initial rainfall events on or near the day of application will often contribute most to annual pesticide lost. In these cases, single event analysis may be required. Procedures are demonstrated for expressing annual total pesticide losses and single rainfall event losses probabilistically in terms of expected recurrence intervals.

Field experiments were conducted from 1996 to 2000 near Manhattan, KS, to determine the effects of application timing on atrazine loss in surface water runoff. In addition, Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) was run to compare simulated loss with actual loss in the field. Atrazine treatments were fall plus preemergence (FALL + PRE), early preplant plus PRE (EPP + PRE), PRE at a low rate (PRE-LOW), and PRE at a full (recommended) rate (PRE-FULL). Ridge-till furrows served as mini watersheds for the collection of surface water runoff. Water runoff volumes and herbicide concentrations were determined for each runoff event. Across four sampling years, mean atrazine runoff loss was 1.7, 4.3, and 1.7% of applied for FALL + PRE, EPP + PRE, and the mean of the PRE treatments, respectively. Thus, actual average losses from FALL + PRE and EPP + PRE treatments were somewhat higher than that predicted by GLEAMS. For PRE treatments, actual average losses were significantly lower than that predicted by GLEAMS, with measured losses falling below the bottom of the graph in 3 of 4 yr. These findings suggest that in certain parts of the Great Plains, FALL + PRE split applications of atrazine offer acceptably low atrazine runoff loss potential; EPP + PRE is more vulnerable to loss than FALL + PRE; and the GLEAMS model may overestimate atrazine runoff potential for PRE applications.