Objective  Spatiotemporal variability of available soil water storage (ASWS) increases the difficulty in water resource management and vegetation restoration in the Chinese Loess Plateau. The aims were to explore the spatial variation of ASWS and analyze the temporal stability and estimation accuracy of ASWS.

Methods  A typical fragmented area in Liudaogou catchment was selected and the long-term data of soil moisture was monitored. Using temporal stability method, the ASWS contents in deep soil layer and soil profile were estimated through the representative soil layers.

Results  The vertical stratification of ASWS was characterized by somewhat diversity and complexity. No relatively stable layer was observed in D < 10 m, while relatively stable layers were 400 - 940 cm and 600 - 900 cm for 10 m < D < 20 m and D > 20 m, respectively. The distribution of ASWS in each soil layer was strongly affected by the ASWS variation in deep soil layers (D < 10 m). The highly significant positive correlation was observed below 100 cm soil layer (r = 0.98, P < 0.001). The mean Spearman rank correlation coefficient among different months was 0.91 for 10 m < D < 20 m, which indicated that the spatial pattern of ASWS showed strong temporal stability. The estimation error and deviation of ASWS were small and estimation equation was highly reliable in 10 m < D < 20 m. The absolute errors between the estimated and measured values of deep ASWS and soil profile ASWS were -1.45 mm and -9.17 mm. Nash efficiency coefficients were 0.75 and 0.78.

Conclusion  Gully affected the accuracy and reliability of deep soil moisture estimation. The most optimal estimation effect was exhibited in 10 m < D < 20 m, followed by D < 10 m, while the estimation performance was slightly worse in D > 20 m. The research results can provide an effective reference for the estimation of deep soil moisture in the fragmented terrain area on the Chinese Loess Plateau.