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ICorrect simulation of overwinter condition is important for the growth of winter crops and for initial growth of spring crops. The objective of this study was to investigate overwinter soil water and temperature dynamics with the simultaneous heat and water (SHAW) model and with its linkage to the root zone water quality model (RZWQM), a hybrid model of RZWQM and SHAW (RZ-SHAW) in a Siberian wildrye grassland under two irrigation treatments (non-irrigation and pre-winter irrigation) in two seasons (2005-2006 and 2006-2007). Experimental results showed that pre-winter irrigation considerably increased soil water content for the top 60-cm soil profile in tbe following spring, but had little effect on soil temperature. Both SHAW and RZ-SHAW simulated these irrigation effects equally well, which demonstrated a correct linkage between RZWQM and SHAW. Across the treatments and years, the average root mean square deviation (RMSD) for simulated total soil water content (liquid plus frozen) was 0.031 m^ m~^ for both RZ-SHAW and SHAW models, and that for liquid water content alone was 0.028 m-^ m~^ for both models. Both models provided better simulation of total and liquid soil water contents under non-irrigation condition than under pre-winter irrigation conditions. On average, RZ-SHAW simulated soil temperature slightly better with an average RMSD of 1.4°C compared to that of 1.8°C by SHAW. Both RZ-SHAW and SHAW simulated the soil freezing process well, but were less accurate in simulating the soil thawing processes, where further improvements are desirable. These simulation results show that the SHAW model is correctly implemented in RZWQM (RZ-SHAW), which adds the capability of RZWQM in simulating overwinter soil conditions that are critical for winter crops.Abbreviations: CK, non-irrigation treatment; ET, evapotranspiration; MF, model efficiency; RMSD, root mean square deviation; RZ-SHAW, a hybrid model of the root zone water quality model and the simultaneous heat and water model; RZWQM, the root zone water quality model; SHAW, the Simultaneous Heat and Water model; TDR, time domain reflectometry; Wl, irrigation treatment before winter.

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