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The Soil Water Assessment Tool (SWAT) model was calibrated for a small watershed with extensive irrigation, low runoff, and a short (2004)(2005)(2006)(2007) period of record in the semi-arid region of the Canadian prairies. The watershed is located on a segment of a river. The net flow between the upstream (inlet) and downstream (outlet) locations of the reach comprises only a fraction of the streamflow at the outlet. This type of watershed is rarely investigated and generally different from the ''pear shaped'' single outlet watersheds presented in SWAT literature. The objectives of this study were to determine if SWAT could be calibrated and validated on this unique watershed using one-and two-way calibration-validation methods, and to determine the effect of a variable climatic record on calibrated and validated parameters. To study the effect of a variable climatic record on the calibrated parameters for this watershed a two-way calibration verification scheme that involved two scenarios was used. Scenario 1 involved calibration using 2004 (average precipitation) and 2005 (above average precipitation) and validation on 2006 and 2007 (both below average precipitation). For scenario 2, the calibration and validation periods were reversed. A third scenario utilized the period of 2004Á2006 for calibration and reserved the 2007 data for validation. The objective criteria used to evaluate model performance included decomposition of the Root Mean Square Error (RMSE) into systematic and unsystematic error, the Nash-Sutcliffe efficiency coefficient (EF) and the coefficient of determination (R 2 ) as model best fit objective functions. The calibration results were satisfactory for scenarios 1 and 3. The calibration of scenario 2 and the validation results for all three scenarios were unsatisfactory. The objective criteria calculated separately for each year in the calibration records showed the calibration results were satisfactory for 2005 only, which is the only wet year in the observed record with above average precipitation. Thus, calibrated parameters were mainly influenced by the 2005 wet season. Therefore, SWAT model calibration on a mixed record of wet and dry conditions will reflect the hydrological processes dominant during wet conditions rather than those observed during dry ones.

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BackgroundThe Zarqa River (ZR) is located in the northern part of Jordan and supplies King Talal Dam (KTD). The streamflow that discharges into KTD is composed of treated wastewater from the Khirbat Es-Samra water treatment plant (KTP) and runoff generated during the winter season. Thus, during the summer, the streamflow of the ZR is dominated by effluent from the KTP. Due to the severe scarcity of water in Jordan, a portion of the streamflow is utilized for irrigated agriculture in the ZR valley, located between the KTP and KTD. The groundwater in the vicinity of the ZR is vulnerable to contamination—a risk that may be exacerbated by the potential occurrence of preferential flow (PF). Therefore, the PF in the soils near the ZR should be carefully considered.MethodsThe macropore flux fraction (Qmacro) and macroscopic capillary length (λc) were determined from in situ measurements using a tension infiltrometer equipped with an infiltration disc with a diameter of 20 cm. The macropore was defined as the pore size that drains at a tension of less than —-3— cm. The λc less than 80 mm was considered to be an indication of PF. The measurements were taken at 69 sites along the ZR between the KTP and KTD. At each measurement site, the soil organic matter content (OM) and soil texture were determined using a composite soil sample obtained by excavating the soil beneath the infiltration disc to a depth of 10 cm.ResultsThe data was split into two groups: the matrix flow group (MF), which includes data associated with λc > 80 mm, and the PF group, which includes data associated with λc < 80 mm. The Qmacro values of 0.67 and 0.57, respectively, for PF and MF were significantly different at p < 0.01 (t-test). The flow rates at h=0 were generally well associated with λc, as attested to by a significant difference between the averages of PF (57.8 mm/hr) and MF (21.0 mm/hr) at p < 0.01 (t-test). The OM was positively associated with PF. This was statistically confirmed by a t-test at p < 0.01. The average sand and clay contents of PF and MF were not statistically different. Analysis of the ratio of Soil Organic Carbon (SOC) to clay showed that the average SOC/clay of the PF (14%) was larger than that of the MF (13.3%). After the exclusion of soils with clay content less than 8%, the differences between the SOC/clay averages of PF (9.8%) and MF (7.5%) were significant at p < 0.05, as shown by a WM-test.ConclusionThe OM was positively associated with PF. Soil texture—and clay content in particular—influenced the λcvalues. However, the association of clay content with PF was not statistically significant. Consideration of the SOC/clay ratio showed that the tendency toward PF increases as the complexation of the clay content increases. This was most obvious in soils with a clay content of greater than 8% and SOC/clay of approximately 10%. The OM either influences or is inter-correlated with the processes responsible for the formation of macropores.