HUC	Count	Mean	Std Dev	Min	Max	HUC	Count	Mean	Std Dev	Min	Max
4	65560	59	13	21	51	59	68	90	12	65784	21	11	2	13	18	26	63
5	65968	36	7	19	32	36	40	67	13	56139	34	19	1	18	32	50	81
6	57642	44	10	15	37	42	51	68	14	65968	61	8	24	57	61	66	80
7	65968	54	10	24	49	54	62	78	15	65967	42	17	7	30	40	53	81
8	65962	31	6	15	26	29	36	54	16	65968	68	7	41	63	67	73	88
9	29568	40	14	14	25	40	53	71	17	65966	69	8	18	67	70	74	87
18	65927	50	14	23	37	51	62	85

Lastly, a third recommendation that has been adopted into the Bin 3 and 4 simulations provides adjustments that are intended to account for how long it takes moving water to transport pesticide to the end of the end of the downstream boundary of the watershed from different starting points in the watershed. A pesticide that is deposited (via runoff or spray drift) into a headwater stream may not reach a downstream waterbody of interest until days later, while a pesticide deposited directly into the downstream waterbody is present immediately. The watersheds of Bins 3 and 4 are sufficiently large that their stream drainage networks include upstream zones that take multiple days for the pesticide and water to be transported to the end of the bin. To account for this, “time-of-travel” adjustments have been implemented, so that the watersheds associated with Bin 3 and 4 waterbodies are divided into fractions that represent the nominal number of days required to move the pesticide through the stream network of the bin. Portions of the total pesticide load (mass) introduced by a runoff or drift event are offset by time lags that reflect their nominal distance, in days, upstream from the end of the bin. Following apportionment by watershed area fraction and offsetting by the appropriate time lag, the time series from each upstream section are superimposed to generate an overall time series reflecting circumstances at the end of the bin. The time series for the runoff volume (flow) and pesticide mass are each treated in this manner. At this time hydrodynamic dispersion, or the flattening of a pesticide concentration in the direction of the flow, is not included in the simulations, although this modification of the new methodology is being considered for future BEs. Representative area fractions for Bin 3 and 4 watersheds are based on mean upstream area fractions within each HUC 2, from NHD data for reaches m3/s, that have mean flowrates within 5% of the defined flowrates for Bins 3 and 4 (1 and 100 respectively). It should be noted that, as this methodology is still under development, this refinement has not been incorporated into the BEs for the three OPs; however, EPA intends to incorporate this adjustment into the draft BEs for carbaryl and methomyl and will update this section of the draft BEs accordingly.

### 3.2. Pesticide Flooded Application Model (PFAM)

PFAM is a model developed specifically for regulatory applications to estimate exposure for pesticides used in flooded agriculture such as rice paddies and cranberry bogs. The model considers the environmental fate properties of pesticides and allows for the specifications of common management practices that are associated with flooded agriculture, such as scheduled water releases and refills. It estimates both acute and chronic concentrations over different durations, allows for defining different receiving water bodies, and allows for more flexibility in refinement of assessments when needed. PFAM allows for the simulation of pesticide applications to a dry field and degradation in soil before water is introduced to the field.