The Impact of US Biofuels Policy on Agricultural Production and Nitrogen Loads in Alabama

Table 1

Hydrological model calibration: parameters.

Parameter	Default values		SUFI-2 estimated values
	Min	Max	New min	New max	Fitted value	𝑃 value^a (sensitivity analysis)
CN2	57.000^b	80.000^b	52.900^b	73.800^b	—	0.000***
ALPHA_BNK	0.000	1.000	0.356	1.074	0.715	0.000***
HRU_SLP	0.000^b	0.400^b	0.000^b	0.523^b	—	0.005***
Precipitation (Hester Creek)	0.000^c	179.830^c	0.000^c	92.250^c	—	0.007***
ESCO	0.010	0.100	0.046	0.119	0.082	0.020**
Precipitation (Paint Rock)	0.000^c	152.950^c	0.000^c	466.500^c	—	0.023**
GW_DELAY	0.000	500.000	−89.328	304.328	107.500	0.034**
RCHRG_DP	0.000	1.000	0.001	0.669	0.335	0.075*
Precipitation (Elk River)	0.000^c	179.320^c	0.000^c	542.440^c	—	0.182
GWQMN	0.000	5000.000	−2017.763	2667.763	325.000	0.190
EPCO	0.010	1.000	−0.182	0.608	0.213	0.217
SLSUBBSN	0.785^b	109.820^b	0.720^b	100.750^b	—	0.233
CH_K(2)	5.000	130.000	31.424	97.326	64.375	0.372
ALPHA_BF	0.100	1.000	0.461	1.188	0.825	0.419
REVAPMN	0.000	500.000	105.690	369.310	237.500	0.452
CH_N(2)	0.000	0.300	0.108	0.327	0.218	0.468
GW_REVAP	0.020	0.100	0.047	0.100	0.073	0.698
CANMX^d	−1.000	1.000	−0.357	0.937	0.290	0.859
SURLAG	0.000	20.000	−9.272	10.272	0.500	0.925

CN2: initial Soil Conservation Service runoff curve number, function of land use, soil’s permeability, and previous soil water condition; ALPHA_BNK: baseflow alpha factor for bank storage (days); HRU_SLP: average slope steepness (m/m); precipitation: rainfall at monitoring point (mm); ESCO: soil evaporation compensation factor; GW_DELAY: groundwater delay time (days); RCHRG_DP: deep aquifer percolation fraction; GWQMN: threshold dept of water in shallow aquifers (mm H₂O); EPCO: plant uptake compensation factor; SLSUBBSN: average slope length (m); CH_K(2): effective hydraulic conductivity (mm/hrs); ALPHA_BF: baseflow alpha factor (days); REVAPMN: threshold dept of shallow aquifers evaporation/deep aquifer percolation (mm H₂O); CH_N(2): Manning’s roughness coefficient for channel flow; GW_REVAP: groundwater evaporation coefficient; CANMX: maximum canopy storage (mm H₂O); SURLAG: surface runoff lag coefficient. ***99%, **95%, and *90% confidence intervals. ^aprobability of the t-statistic of the coefficients obtained by regressing the Latin hypercube parameters, generated during the calibration procedure and against the objective function values [23]; ^bminimum and maximum value of the HRU sample of parameters; ^cminimum and maximum value of measured and resampled precipitation at monitoring points; ^dthe fitted value is summed to one and multiplied by the default value (percentage change).