Estimation of phosphorus export from a Mediterranean agricultural catchment with scarce data

Figures & data

Fig. 1 The Enxoe catchment and location of the sampling sites.

Table 1 Catchments neighbouring the Enxoe catchment, their available daily discharge data time span and water balance. d_Enxoe: distance to Enxoe catchment; P: precipitation; Q: mean discharge; C_Q: runoff fraction; ET_a: calculated storage change, including evapotranspiration; C_ET: ET_a as a fraction of Q. Values in brackets are standard deviations.

Catchment	Area (km^2)	dEnxoe (km)	Discharge data time span	P (mm)	Q (mm)	CQ (%)	ETa (mm)	CET (%)
Entradas	51	58	1971–2000	489 (162)	116 (134)	24	373 (85)	76
Limas	249	25	1982–1990	519 (202)	89 (101)	17	430 (114)	83
Monte da Arregota	97	38	1981–1990	529 (170)	95 (126)	18	434 (85)	82
Monte da Ponte	618	42	1971–1997	482 (163)	127 (135)	26	355 (74)	74

Table 2 Parameters, their units and calibrated values of the ZIN-Sed 2D model in this study. P: phosphorus.

Parameter	Unit	Description	Value
Kf1_m	–	Multiplier of hydraulic conductivity of soil layer 1	3
Kf23_m	–	Multiplier of hydraulic conductivity of soil layers 2 and 3	2
soilD_m	–	Multiplier of soil depth	0.4
Klat_m	–	Anisotropy factor in soil	5
Crop_m	–	Multiplier of FAO crop coefficient	0.9
τc,erosion	N m^-2	Critical shear stress for erosion in overland flow	0.15
τc,depo	N m^-2	Critical shear stress for deposition in overland flow	0.10
EC_OF	g m^-2 s^-1	Erosion coefficient in overland flow	0.003
VS_OF	mm s^-1	Settling velocity of sediments in overland flow	0.1
TC_OF	–	Transport capacity factor in overland flow	0.2
TC_River	–	Transport capacity factor in river	0.2
Dm	–	Multiplier of rainfall detachability factor	0.1
KL	L mg^-1	Parameter K of Langmuir isotherm	1.3
cmax	mg g^-1	Maximum sorbed concentration	3.5
rad	d^-1	Rate of adsorption	20
rde	d^-1	Rate of desorption	20
SPm	–	Multiplier of soil P content	4
b	–	Exponent of initial soil P profile	–0.5

Fig. 2 Results of the local sensitivity analysis. Parameter abbreviations are explained in Table 2. The ZIN-Sed 2D modules (hydrology, erosion, phosphorus) of the parameters are indicated above the diagram. SSC: suspended sediment; PP: particulate phosphorus; and DP: dissolved phosphorus.

Table 3 Quantiles (Q_x at x percent), regression slope (s), coefficient of determination (R²) and discharge values (Q_SFCD) of the synthetic flow duration curve. Regressions were made with n = 4 catchments (Table 1).

	R^2	s	QSFCD (m^3 s^-1)
Q0.05	0.96	0.34608	12.86
Q0.1	0.95	0.29550	10.98
Q0.5	0.96	0.14317	5.321
Q1.0	0.95	0.09815	3.648
Q2.5	0.96	0.03576	1.329
Q5.0	0.98	0.01325	0.492
Q10	1.00	0.00384	0.143
Q15	0.98	0.00142	0.053
Q20	0.98	0.00084	0.031
Q30	0.99	0.00028	0.010
Q40	0.92	0.00009	0.003
Q50	0.65	0.00002	0.001

Fig. 3 Synthetic and modelled flow duration curve of the Enxoe River.

Fig. 4 Seasonal discharge (Q) regimes as sampled in catchments neighbouring the Enxoe (ranges) and modelled for the Enxoe catchment by ZIN-Sed 2D (circles).

Fig. 5 Modelled and sampled mean event concentrations of suspended sediment (SSC), dissolved (DP), sorbed (PP) and total (TP) phosphorus. The grey, dashed line denotes the 1:1 line. RMSE: root mean square error; R²: coefficient of determination with n = 8 values.

Fig. 6 Modelled and sampled baseflow concentrations of dissolved phosphorus (DP). The sampled DP range is the range between sampled values of the two tributaries.

Fig. 7 Long-term assessment of mean annual suspended sediment (SSY), dissolved (DP) and particulate (PP) phosphorus release, discharge and total phosphorus (TP) concentrations from the Enxoe catchment.