Observation of water and solute movement in a saline, bare soil, groundwater seepage area, Western Australia. Part 1: Movement of water in near-surface soils in summer Eiichi Shimojima A,D , Ichiro Tamagawa B , Masato Horiuchi A , Robert J. Woodbury C , and Jeffrey V. Turner C A Daido University, 40 Hakusui-cho, Minami-ku, Nagoya, 457-8532, Japan. B River Basin Research Center, Gifu University, 1-1 Yanado, Gifu, 501-1193, Japan. C CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia. D Corresponding author. Emails: shimoji@daido-it.ac.jp; eiichi-s@kyoto.zaq.jp Abstract. In order to elucidate the relationship between evaporation, salinisation, and annual water and salt balances in semi-arid and arid regions, hydrological and meteorological observations were undertaken over 3 years in a small, salinised, bare-soil, groundwater seepage area in Western Australia. This paper focuses on water behaviour near a bare saline soil surface during the dry summer. Analysis of observed data on soil vapour density using a vapour diffusion transfer model can account for the daily upward vapour flux from the soil surface that occurs in midsummer. The dry soil undergoes cycles of drying during the day, accompanied by salt crust formation and wetting during the night. In late summer, the same zones show a wetting trend owing to a marked atmospheric vapour invasion and condensation at night regardless of evaporation during daytime. The daily average vapour flux at the ground surface in mid- and late-summer, respectively, estimated through the vapour transfer model in the dry soil layer was ~0.35 and 0.03 mm/day. Comparison of vapour fluxes at the ground surface measured with a portable surface evaporimeter with modelled estimates of vapour transport in soil showed agreement of the proposed model to field results at low wind speed, but not at the higher wind speeds. This identifies the active role of turbulent surface wind speed on vapour transfer in the dry soil layer below the ground surface. Additional keywords: evaporation, condensation, vapour transport, salt crust, semi-arid area. Received 9 March 2012, accepted 27 June 2013, published online฀2฀September฀2013 Introduction In semi-arid and arid regions, water resources are under serious threat due to water scarcity and deterioration of quality due to salinity (e.g. Peck and Hatton 2003). Furthermore, in low-lying parts of the landscape where groundwater seepage occurs, salinity can affect productive land-use as well as ecosystem functioning. Understanding of salt and water behaviour in groundwater seepage areas and the detail of discharge mechanisms is necessary to sustain productive activity and the survival of native vegetation as well as to determine land management practices that will reduce the area of salinised land. Dryland salinity in Western Australia has received considerable attention over many years (Short and McConnell 2000; and references therein). To date, however, there have been only a few investigations on the water and solute fluxes and mass balances at the length scale of a saline seepage area, i.e. tens of meters in the horizontal and a few meters in the vertical scale (George and Conacher 1993a, 1993b; Bennett et al. 2012). This research attempts to develop an understanding of the governing physical, hydrological, and meteorological processes of groundwater seepage and salinisation, with the expectation that the principles are characteristic of many similar salinised seepage areas in the south-west of Western Australia. Evaporation from bare soil causes upward solute transport in the soil profile and salt tends to accumulate near to the ground surface, while the evaporation rate is in turn reduced by salt accumulation in the drying soil. Thus, evaporation and salinisation are closely correlated. The mechanisms of these mutual interactions have been investigated in laboratory experiments (Shimojima et al. 1996), numerically (e.g. Bittelli et al. 2008), and by field observations (Salhotra et al. 1985; Ullman 1985; Woods et al. 1990; Thorburn et al. 1992; Grünberger et al. 2008). Difficulties encountered in field studies include making the appropriate physical measurements and the extrapolation of localised results with high spatial variability to larger areas. Specialised instruments and methods are required to estimate low soil evaporation and condensation rates in the field, which then allow the analysis of complex mass exchange and transfer mechanisms near the land–atmosphere boundary. When the atmospheric evaporation demand is greater than the ability of the soil to conduct soil water, drying of the surface soil layer occurs, accompanied by a downward migration of an Journal compilation Ó CSIRO 2013 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2013, 51, 288–300 http://dx.doi.org/10.1071/SR12282