Theoretical considerations and mathematical tools to analyze multidimensional transient infiltration from a trickle source have been developed. Two mathematical models were considered: (i) a plane flow model involving the Cartesian coordinates x and z; and (ii) a cylindrical flow model described by the cylindrical coordinates r and z. The diffusion‐ type water flow equation in unsaturated soil was solved numerically by an approach that combines the noniterative ADI difference procedure with Newton's iterative method. The numerical results have been compared with Wooding's solution for steady infiltration from a circular pond and with a simple one‐dimensional solution. These comparisons indicate that the method is reliable and can be used with confidence. Typical results are demonstrated to show the effect of trickle discharge on the water content field, the saturated water entry zone, and the water flux at the soil surface.

The theory of transient infiltration from a trickle source, as developed in Part I of this work, was compared with experimental results. Laboratory experiments using a loamy soil were conducted under conditions similar to those assumed in the two‐dimensional plane flow model. Field data were collected from a sandy soil that was wetted by commercial trickles. The field conditions were similar to those assumed in the cylindrical model. The effect of trickle discharge rates on the water content distribution and on the location of the wetting front was considered. The functional relationship between water diffusivity and water content was estimated by a computer technique that complements the conventional method for diffusivity determination. These data, together with the soil water retention curve were used to estimate the hydraulic conductivity as a function of water content. The agreement between theory and experiment, as expressed by water content distribution and location of the wetting front, is generally good and suggests that the theory is applicable to many field situations. Significant discrepancies between observed and theoretical results were obtained only in one case, where the rate of trickle infiltration was large. The theory, as well as the experimental data, indicate that for the conditions studied, an increase in the trickle discharge rate results in an increase in the horizontal wetted area and a decrease in the soil wetted depth.

Warm winters and high precipitation in north‐eastern Japan generate snow covers of more than three meters depth and densities of up to 0.55 g cm−3. Under these conditions, rain/snow ratio and snowmelt have increased significantly in the last decade under increasing warm winters. This study aims at understanding the effect of rain‐on‐snow and snowmelt on soil moisture under thick snow covers in mid‐winter, taking into account that snowmelt in spring is an important source of water for forests and agriculture. The study combines three components of the Hydrosphere (precipitation, snow cover and soil moisture) in order to trace water mobility in winter, since soil temperatures remained positive in winter at nearly 0.3°C. The results showed that soil moisture increased after snowmelt and especially after rain‐on‐snow events in mid‐winter 2018/2019. Rain‐on‐snow events were firstly buffered by fresh snow, increasing the snow water equivalent (SWE), followed by water soil infiltration once the water storage capacity of the snowpack was reached. The largest increase of soil moisture was 2.35 vol%. Early snowmelt increased soil moisture with rates between 0.02 and 0.035 vol% hr−1 while, rain‐on‐snow events infiltrated snow and soil faster than snowmelt and resulted in rates of up to 1.06 vol% hr−1. These results showed the strong connection of rain, snow and soil in winter and introduce possible hydrological scenarios in the forest ecosystems of the heavy snowfall regions of north‐eastern Japan. Effects of rain‐on‐snow events and snowmelt on soil moisture were estimated for the period 2012–2018. Rain/snow ratio showed that only 30% of the total precipitation in the winter season 2011/2012 was rain events while it was 50% for the winter 2018/2019. Increasing climate warming and weakening of the Siberian winter monsoons will probably increase rain/snow ratio and the number of rain‐on‐snow events in the near future.

Stable isotopes of xylem water ( 18 O and D) have been successfully used to determine sources of soil water for plant transpiration, but mainly in drought-prone environments. The water uptake strategies of three representative tree species in Japan, namely cedar (Cryptomeria japonica), larch (Larix kaempferi) and beech (Fagus crenata), were investigated using δ 18 O and δD of water (precipitation, soil and xylem), together with wood α-cellulose δ 13 C and δ 18 O, along one growing season. The study was carried out in the research forest of Yamagata University (Shonai region), a high precipitation area in Japan, which exceeds 3000 mm per year. Precipitation water δ 18 O and δD increased along the summer growing season, but oxygen and hydrogen isotopic composition of soil water remained essentially unchanged. In general, xylem water isotopes of cedar and larch followed the local meteoric water line, but beech xylem water was decoupled from soil and precipitation values in July and August. For this tree species, the xylem water isotopic records were more depleted than cedar and larch xylem water isotopic values and the precipitation water isotopic records, indicating that beech used more water from soil layers during July-August than the other two species, which mainly used newly-fallen precipitation. Wood δ 18 O showed an opposite seasonal trend to the one found for xylem water, likely because of leaf water isotope enrichment, which was in turn controlled by seasonal transpiration rate. The higher δ 13 C values of cedar during summer suggested that this species had enhanced water-use efficiency during the growing season compared with the deciduous species larch and beech. Our results highlight different water use strategies among forest tree species even in areas where the annual water balance is far from limiting plant performance.