perspective, daily mean concentrations provide a more meaningful metric, than do initial concentrations, for comparison against the results of acute toxicity studies, where organisms are exposed to a pesticide for at least 48 hours. Additionally, the initial concentrations are essentially a hypothetical construct that is inherent in the way EFED currently calculates daily mean concentrations. When a modeled receiving water body is relatively large compared with its watershed and has a residence time (i.e., the amount of time water spends in a waterbody) on the order of months, which is the case for EPA’s Index Reservoir, or is assumed to have no flow out of it, which is the case for EPA’s standard pond, daily mean and initial concentrations are typically very close to each other. However, as the size (volume) of receiving water body shrinks in comparison to its watershed, and particularly as residence times decline below one day, as is the case for the flowing bins used in the BEs, the initial and daily mean concentrations begin to diverge, with smaller residence times resulting in a larger divergence. This is a result of the assumptions inherent in the calculation methodology, and not a reflection of fundamental differences in physical or chemical processes at work in smaller receiving water bodies.

Another recommendation that has been adopted is incorporation of baseflow into the flow for Bins 3 and 4. Baseflow is the portion of streamflow that comes from subsurface discharge to a stream or river (as opposed to direct overland runoff). Baseflows for use in modeling were derived from regionally- representative estimates of baseflow index (BFI), defined as the fraction of total (long-term) stream flow that consists of baseflow. BFI values were extracted from EPA’s Stream-Catchment (StreamCat) dataset, which provides estimates of this property for the millions of flowing reaches across the country that are represented in the National Hydrography Dataset (NHD). Within each HUC 2, the average BFIs for reaches with flows similar to those of Bins 3 and 4 were calculated and tabulated (Table A 3-1.15). The m3/s HUC/Bin specific BFIs are currently applied to the annual average flowrate assigned for each bin (1 m3/s and 100 for Bin 3 and 4, respectively), in order to provide a HUC 2-specific baseflow value for use in each simulation. For HUC 3, BFI data are available for three separate regions in NHDPlus: South Atlantic North (3N), South Atlantic South (3S), and South Atlantic West (3W). As the aquatic modeling for HUC 3 is not subdivided into a and b categories to account for changes in precipitation, the region with the lowest median BFI, 3N, was used, as it would generate lower baseflow estimates and more protective EECs. Similarly, for HUC 10, data are available for two separate regions in NHDPlus: the Upper Missouri (10U) and Lower Missouri (10L). As the aquatic modeling for HUC 10 is subdivided into a and b categories to account for changes in precipitation, a simplified map analysis was used to assign the BFI for 10L to modeling for HUC 10a and the BFI for 10U to modeling for HUC 10b. Lastly, it should be noted that BFI data are not available for HUCs 19 (AK), 20 (HI), and 21 (PR). As such, BFI data for nearby surrogate regions with similar meteorological conditions (HUC 17 for HUCs 19 and HUC 20 and HUC 3 for HUC 21) were used until such time as data become available for these regions.

###### Table A 3-1.15 Baseflow Index Statistics for Each HUC 2

HUC	Count	Mean	Std Dev	Min	25%	50%	75%	Max	HUC	Count	Mean	Std Dev	Min	25%	50%	75%	Max
1	65368	51	4	40	48	50	54	72	10a	65968	52	14	17	41	51	61	86
2	65549	48	10	25	42	47	53	88	10b	65965	57	21	10	42	67	72	86
3	65395	42	9	25	35	40	48	67	11	65968	35	13	6	25	35	44	76