Increase In Precipitation Intensity Research Articles

Assessing the impact of climate change on the land hydrology in Taiwan

The gradually increased temperature resulting from the enhanced greenhouse effects has been found to be an important factor of changes to the global climate which in turn might significantly affect the Earth's hydrological cycles. The possible outcomes of warming climate are changes of precipitation, surface runoff, evapotranspiration, and frequency of extreme weather events, such as floods and droughts. However, such changes at the global scale may not reflect the variations on a regional scale, and more so at the local scale. In this study, a physically based water balance model was applied to study the impact of climate change on the land hydrology, focusing on trends of surface runoff, evapotranspiration, and infiltration in Taiwan. Model forcing of composite temperatures and precipitations were generated by a weather generation model in association with nine climate change scenarios, including outputs of equilibrium experiments and special reports on emissions scenarios, from the IPCC. Although discrepancies among different climate change scenarios are significant, the trend of more extreme precipitations and surface runoffs were observed in most scenarios' runs. The increase of evapotranspiration in both wet and dry seasons is persistent among different scenarios throughout the island due to the projected consistently higher temperature. Although the trends of infiltration for wet and dry seasons are opposite in curtain scenarios, a decreased yearly infiltration was found in most cases as the result of increased precipitation intensity and more evapotranspiration. Timely adaption measures for water resources managements and natural hazard mitigations are required to face these changes of land hydrology components under changing climate.

Holocene changes in eastern tropical Pacific climate inferred from a Galápagos lake sediment record

Paleoclimate records from the tropical Pacific suggest the early to mid-Holocene was a period of reduced El Niño/Southern Oscillation (ENSO) variability, with a transition to modern, increased ENSO frequency occurring some time in the last few thousand years. However, the nature and timing of this shift remains uncertain due to the discontinuous nature and/or coarse resolution of many ENSO proxies, as well as a lack of agreement between previously published records. A new, continuous, climate record from El Junco Crater Lake in the Galápagos Islands reveals several abrupt changes in lake level and precipitation through the Holocene. Hydroclimatic model simulations suggest that El Junco lake level responds sensitively to increases in precipitation associated with El Niño events, rising during wet El Niño events and falling during the intervening dry periods. Grain size data from El Junco sediment cores indicate past lake level variability, likely associated with changing seasonal precipitation and ENSO frequency. The grain size data suggest increased precipitation intensity prior to 9000±120 cal years BP, and after 4200±130 cal years BP, as well as a two-step increase in precipitation at 3200±160 and 2000±100 cal years BP. Maximum Holocene precipitation and inferred ENSO variability occurred between 2000±100 and 1500±70 cal years BP, during the same period that six other independent proxy records suggest higher ENSO frequency and longer, stronger El Niño events. Decreasing sediment carbon/nitrogen (C/N) ratios in El Junco sediments indicate rising lake levels from the early Holocene to present, corroborating the grain size data. The inferred increase in precipitation at 4200±130 cal years BP and at 2000±100 cal years BP coincides with decreasing Southwest Asian and East Asian Monsoon intensity, suggesting tropical Pacific climate and the Asian monsoon were interconnected systems at centennial to millennial timescales during the Holocene. A weakening trend in the Asian monsoon and the trend toward wetter conditions at El Junco also coincide with a trend toward cooler and drier conditions inferred from Cariaco Basin sediment proxies from the mid-Holocene to present, suggesting the migration of the Intertropical Convergence Zone (ITCZ) likely influenced hydrological changes in both the eastern tropical Pacific and the Asian Monsoon region during the Holocene.

Effects of paleoclimate and time‐varying canopy structures on paleowater fluxes

We combined a long‐term (18,000 years) climatological record and time‐varying vegetation conditions to evaluate the role that climate change and vegetation might play in paleowater fluxes in arid settings. The HYDRUS‐1D model, which solves Richards Equation for variably saturated flow, the convection‐dispersion equation for chloride transport and the heat flow equation, was used to simulate water flux and chloride (Cl) transport. Six distinct case studies were compared for different boundary conditions and root distributions. A Mojave Desert–type canopy including evergreens, drought deciduous shrubs, annuals, grasses, and succulents was used as representative vegetation to transpire soil water and was modeled using ground cover percentage and leaf area index (LAI) as the bases for partitioning evapotranspiration (ET). The results showed that under water limited conditions, realistic root zone distributions and climate sequences (including extreme events) were both needed to simulate the accumulation of Cl in Mojave Desert soils. Results also showed that increasing precipitation intensity affected paleowater fluxes. However, contrary to the results of other researchers, we found that simulated chloride bulges were located at depths of around 20–30 cm, rather than at the base of the root zone, if current and normal climate conditions were applied. Moreover, the climatic shift beginning in the late Pleistocene was not the major reason for the chloride accumulation.

United Kingdom daily precipitation intensity: improved early data, error estimates and an update from 2000 to 2006

AbstractThis paper updates the analysis by Osborn et al. of trends in the contribution of heavy events to precipitation in the UK. We spatially extended the previous analysis of 110 rain gauges to a set of 689 rain gauges covering almost the whole UK, and updated the results to November 2006. For each station and season, we calculated ten time series of the contribution of ten precipitation amount categories to the total seasonal precipitation. A principal component analysis of post‐1961 trends of all categories and stations is consistent with earlier results, namely, widespread shifts towards greater contribution from heavier precipitation categories during winter, and towards light and moderate categories during summer. Regional and UK average time series of the contribution from the category consisting of the heaviest events indicate that the increased winter intensity was sustained during the most recent ten years, but the trend did not continue at the rate reported previously for 1961–1995. For summer, the decreasing contribution from the heaviest rainfall category reported for 1961–1995 underwent a reversal during the most recent decade, returning towards the 1961–1995 reference level of intensity. Confidence intervals for these regional and UK average time series were estimated by a bootstrap approach and indicate that the sparser observations from the first half of the 20th century are still sufficient to estimate UK average change. These longer records support the existence of a long‐term increase in winter precipitation intensity, and similar trends have now also become evident in spring and (to a lesser extent) autumn. The summer rainfall intensity has exhibited changes that are more consistent with inter‐decadal variability than any overall trend. Copyright © 2008 Royal Meteorological Society

How Often Will It Rain?

Abstract Daily precipitation data from climate change simulations using the latest generation of coupled climate system models are analyzed for potential future changes in precipitation characteristics. For the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) B1 (a low projection), A1B (a medium projection), and A2 (a high projection) during the twenty-first century, all the models consistently show a shift toward more intense and extreme precipitation for the globe as a whole and over various regions. For both SRES B1 and A2, most models show decreased daily precipitation frequency and all the models show increased daily precipitation intensity. The multimodel averaged percentage increase in the precipitation intensity (2.0% K−1) is larger than the magnitude of the precipitation frequency decrease (−0.7% K−1). However, the shift in precipitation frequency distribution toward extremes results in large increases in very heavy precipitation events (>50 mm day−1), so that for very heavy precipitation, the percentage increase in frequency is much larger than the increase in intensity (31.2% versus 2.4%). The climate model projected increases in daily precipitation intensity are, however, smaller than that based on simple thermodynamics (∼7% K−1). Multimodel ensemble means show that precipitation amount increases during the twenty-first century over high latitudes, as well as over currently wet regions in low- and midlatitudes more than other regions. This increase mostly results from a combination of increased frequency and intensity. Over the dry regions in the subtropics, the precipitation amount generally declines because of decreases in both frequency and intensity. This indicates that wet regions may get wetter and dry regions may become drier mostly because of a simultaneous increase (decrease) of precipitation frequency and intensity.

Potential future changes in the characteristics of daily precipitation in Europe simulated by the HIRHAM regional climate model

In this study the potential future changes in various aspects of daily precipitation events over Europe as a consequence of the anticipated future increase in the atmospheric greenhouse gas concentrations are investigated. This is done by comparing two 3-member ensembles of simulations with the HIRHAM regional climate model for the period 1961–1990 and 2071–2100, respectively. Daily precipitation events are characterized by their frequency and intensity, and heavy precipitation events are described via 30-year return levels of daily precipitation. Further, extended periods with and without rainfall (wet and dry spells) are studied, considering their frequency and length as well as the average and extreme amounts of precipitation accumulated during wet spells, the latter again described via 30-year return levels. The simulations show marked changes in the characteristics of daily precipitation in Europe due to the anticipated greenhouse warming. In winter, for instance, the frequency of wet days is enhanced over most of the European continent except for the region on the Norwegian west coast and the Mediterranean region. The changes in the intensity and the 30-year return level of daily precipitation are characterized by a similar pattern except for central Europe with a tendency of decreased 30-year return levels and increased precipitation intensity. In summer, on the other hand, the frequency of wet days is decreased over most of Europe except for northern Scandinavia and the Baltic Sea region. In contrast, the precipitation intensity and the 30-year return level of daily precipitation are increased over entire Scandinavia, central and eastern Europe. The changes in the 30-year return level of daily precipitation are generally stronger than the corresponding changes in the precipitation intensity but can have opposite signs in some regions. Also the distribution of wet days is changed in the future. During summer, for instance, both the frequency and the length of dry spells are substantially increased over most of the European continent except for the Iberian Peninsula. The frequency and the length of wet spells, on the other hand, are generally reduced during summer and increased during winter, again, with the exception of the Iberian Peninsula. The future changes in the frequency of wet days in winter are related to a change in the large-scale flow over the North Atlantic and a corresponding shift of the North Atlantic storm track. The reduction in the frequency of wet days in summer is related to a northward extension of the dry subtropical region in the future, with a reduction of the convective activity because of the large-scale sinking motion in the downward branch of the Hadley cell. Because the atmosphere contains more moisture in the warmer future climate, the amount of precipitation associated with individual low-pressure systems or with individual convective events is increased, leading to a general increase in the intensity of individual precipitation events. Only in regions, where all the moisture evaporates from the ground already in spring, the intensity of precipitation events is reduced in summer.

The Atlantic multidecadal oscillation and extreme daily precipitation over the US and Mexico during the hurricane season

The tail of the distribution of daily precipitation for August–September–October was examined over the United States and Mexico in relation to the Atlantic Multidecadal Oscillation (AMO). As expected from previous studies linking the AMO to hurricane activity, Florida and the coastal Southeast US showed an increase in precipitation intensity when the Atlantic was in a warm phase (AMO+). Also during AMO+ Northwest Mexico was dry and exhibited a reduction of extreme events and the Mid-Atlantic Appalachian Mountains showed evidence of an increase in heavy precipitation compared to when the Atlantic was cool. It is proposed that the aforementioned decadal variations in extreme rainfall are forced by changes in the large-scale surface winds and air temperature in conjunction with the AMO. Namely, an anomalous cyclonic circulation is observed off the Southeast coast, leading to a reduction of moisture flux into the decaying North American monsoon, and an increase in moisture flux into the Mid-Atlantic. Further, the Mid-Atlantic shows a relatively strong increase in the mid-tropospheric lapse rate. Thus, the unique combination of low-level humidity, potential instability, and elevated topography are consistent with an enhanced risk of intense rainfall during AMO+.

Trends in Seoul (1778–2004) summer precipitation

Precipitation data for Seoul, Korea, have been recorded since 1778, representing one of the world's longest instrumental measurements of daily precipitation. Trends in summer precipitation amount, intensity, and indices of climate extremes are studied. The detected trends were tested using trend‐to‐noise ratio test, nonparametric Mann–Kendall rank statistic and Monte Carlo simulation procedure. Over the past 227 years and in the twentieth century, the June–September total precipitation amount has a trend significant at 95% confidence level; the numbers of torrential rainy days with precipitation greater than 75 mm, the highest one‐day rainfall amount, the number of extremely wet days defined as the 99 percentile of the distribution of daily precipitation, and the precipitation amount falling in the extremely wet days, all have a trend significant at 99% confidence level. An intensity index that captures the combined effects of precipitation frequency and intensity, calculated as the number of heaviest rainy days that constitute 67% of the total precipitation shows a decreasing trend significant at 99% confidence level, suggesting an increase in precipitation intensity. However, the trends calculated for the pre‐1950 period (1778–1949) are all insignificant except the intensity index, suggesting that real trends occur only in the recent 55 years. Nevertheless, the pronounced recent increasing trends since 1950 are unprecedented. What give rise this trend remain elusive.

Understanding future patterns of increased precipitation intensity in climate model simulations

In a future climate warmed by increased greenhouse gases, increases of precipitation intensity do not have a uniform spatial distribution. Here we analyze a multi‐model AOGCM data set to examine processes that produce the geographic pattern of these precipitation intensity changes over land. In the tropics, general increases in water vapor associated with positive SST anomalies in the warmer climate produce increased precipitation intensity over most land areas. In the midlatitudes, the pattern of precipitation intensity increase is related in part to the increased water vapor being carried to areas of mean moisture convergence to produce greater precipitation, as well as to changes in atmospheric circulation. Advective effects, indicated by sea level pressure changes, contribute to greatest precipitation intensity increases (as well as mean precipitation increases) over northwestern and northeastern North America, northern Europe, northern Asia, the east coast of Asia, southeastern Australia, and south‐central South America.

Future projections of precipitation characteristics in East Asia simulated by the MRI CGCM2

Projected changes in precipitation characteristics around the mid-21st century and end-of-the-century are analyzed using the daily precipitation output of the 3-member ensemble Meteorological Research Institute global ocean-atmosphere coupled general circulation model (MRI-CGCM2) simulations under the Special Report on Emissions Scenarios (SRES) A2 and B2 scenarios. It is found that both the frequency and intensity increase in about 40% of the globe, while both the frequency and intensity decrease in about 20% of the globe. These numbers differ only a few percent from decade to decade of the 21st century and between the A2 and B2 scenarios. Over the rest of the globe (about one third), the precipitation frequency decreases but its intensity increases, suggesting a shift of precipitation distribution toward more intense events by global warming. South China is such a region where the summertime wet-day frequency decreases but the precipitation intensity increases. This is related to increased atmospheric moisture content due to global warming and an intensified and more westwardly extended North Pacific subtropical anticyclone, which may be related with an El Nino-like mean sea surface temperature change. On the other hand, a decrease in summer precipitation is noted in North China, thus augmenting a south-to-north precipitation contrast more in the future.

Impact of expected increase in precipitation intensities on soil loss—results of comparative model simulations

The impact of the expected climate change on the frequency and extent of soil erosion processes is hardly assessable so far. This is mainly because available models of climate change reliably produce at best mean daily precipitation data, whereas erosion is the result of extreme but short time rainfall and runoff events, normally lasting no longer than a few hours. The frequency and intensity of these extreme rainfall events are expected to increase in some regions, which could lead to increased erosion rates. Mathematical models are able to describe erosion rates under conditions of these extreme events, however, so far prognostic meteorological data necessary for the application of these models are not available. The use of a new method for the projection of meteorological time series and their extremes using global climate simulations [ Enke, W., Spekat, A., 1997. Downscaling climate model outputs into local and regional weather elements by classification and regression. Climate Research 8, 195–207; Enke, W., 2000. Regionalisierung von Klimamodellergebnissen mittels des statistischen Verfahrens der Wetterlagenklassifikation und nachgeordneter multipler Regression für Sachsen. 5. Deutsche Klimatagung, Hamburg, 2–6. Oktober 2000; Enke, W., 2003. Regionaler Wandel im Freistaat Sachsen. AZ. 13-8802.3521/51, Im Auftrag des Sächsischen Landesamtes für Umwelt und Geologie; Enke, W., Deutschländer, Th., Schneider, F., Küchler, W., 2005. Results of five regional climate studies applying a weather pattern based downscaling method to ECHAM4 climate simulations. Meteorologische Zeitschrift 14, 247–257; Enke, W., Schneider, F., Deutschländer, Th., in press. A novel scheme to derive optimized circulation pattern classifications for downscaling the forecast purposes. Theoretical and Applied Climatology] permits for the first time an approximation of future soil loss. This research is based on simulated, high resolution data for extreme rainfall events in the period of 2031–2050, which reproduces the mean frequency, intensity and duration of future events with high precipitation intensities relevant to erosion within the investigated seasonal period from June to August. The simulations are performed for two exemplary sites in Saxony, based on the EROSION 2D model ( Schmidt, J., 1990. A mathematical model to simulate rainfall erosion, Catena, Suppl. 19), which is a process-based soil erosion model for simulating soil erosion and deposition by water on single slopes. Simulated precipitation for the 2031–2050 time period is used to model soil loss, and results are compared to soil loss based on 20 years of measured precipitation from 1981 to 2000. The simulation results allow the impacts of climate change on erosion rates to be quantified by comparing current climate with predicted, future climate. However, expected changes in land use due to changed economic conditions are not taken into account in this analysis.

Trends in Total Precipitation and Frequency of Daily Precipitation Extremes over China

Abstract Based on a newly developed daily precipitation dataset of 740 stations in China and more robust trend detection techniques, trends in annual and seasonal total precipitation and in extreme daily precipitation, defined as those larger than its 95th percentile for the year, summer, and winter half years, have been assessed for the period 1951–2000. Possible links between changes in total precipitation and frequency of extremes have also been explored. The results indicate that there is little trend in total precipitation for China as a whole, but there are distinctive regional and seasonal patterns of trends. Annual total precipitation has significantly decreased over southern northeast China, north China, and over the Sichuan Basin but significantly increased in western China, the Yangtze River valley, and the southeastern coast. In western China, precipitation increase has been observed for both cold and warm seasons. However, trends differ from one season to another in eastern China. Spring precipitation has increased in southern northeast China and north China but decreased significantly in the midreach of the Yangzte River. The summer precipitation trend is very similar to that of annual totals. Autumn precipitation has generally decreased throughout eastern China. In winter, precipitation has significantly decreased over the northern part of eastern China but increased in the south. The number of rain days has significantly decreased throughout most parts of China with northwest China being an exception. Meanwhile, precipitation intensity has significantly increased. This suggests that the precipitation increase in western China is due to the increase in both precipitation frequency and intensity. In eastern China, the impact of reduced number of rain days seems to be more dominant in the north while the influence of enhanced intensity prevails in the south. Over regions with increasing precipitation trends, there have been much higher than normal frequency of precipitation extreme events. For example, significant increases in extreme precipitation have been found in western China, in the mid–lower reaches of the Yangtze River, and in parts of the southwest and south China coastal area. A significant decrease in extremes is observed in north China and the Sichuan Basin. Trends in the number of extremes and total precipitation from nonextreme events are generally in phase. An exception is southwest China where an increase of extreme events is associated with a decrease in total nonextreme precipitation.

Statistical downscaling and scenario construction of precipitation in Scania, southern Sweden

Statistical downscaling models for precipitation in Scania, southern Sweden, have been developed and applied to calculate the changes in the future Scanian precipitation climate due to projected changes in the atmospheric composition. The models are based on multiple linear regression, linking large-scale predictors at monthly time resolution to regional statistics of daily precipitation on a monthly basis. To account for spatial precipitation variability within the area, the precipitation statistics were derived for different regions in Scania. The final downscaling models, developed for different regions and seasons, use atmospheric circulation, large-scale humidity and precipitation as predictors. Among the precipitation statistics examined, only the models for estimating the mean precipitation and the frequency of wet days were skilful. Based on the Canadian Global Circulation Model 1 (CGCM1), a future scenario of these two statistics was created. The downscaled scenario shows a significant increase of the annual mean precipitation by about 10% and a slight decrease in the frequency of wet days, indicating an increase in the precipitation amounts as well as in the precipitation intensity. The main increase of precipitation amounts and intensity occur during winter, while the summer precipitation amounts decrease slightly. The seasonal changes found in precipitation are likely attributed to changes in the westerly flow of the atmospheric circulation.

Episodic Late Quaternary slopewash deposition as recorded in colluvial aprons, Southeastern Wyoming

Colluvial aprons found along fluvial terraces of the Laramie River and Sybille Creek in southeastern Wyoming are interpreted as late Quaternary slopewash deposits. Each apron studied contained multiple buried soils, interpreted to indicate that slopewash activity was episodic and short-lived, and was followed by relatively long periods of landscape stability and soil development. Apron deposits were described and subsequently correlated based on their internal stratigraphy and their relative degree of soil development. Optical dating was used to chronologically constrain periods of slopewash deposition, and based on 17 optical ages taken from five aprons, aggradation occurred at ∼65–56, 16.0, 12.9, 11.8, 8.1, 7.3, 5.0, and 1.7 ka. Several of these events correspond with climatic transitions such as the termination of Heinrich Event 1, the onset and termination of the Younger Dryas, and the 8.2 ka event, suggesting that apron aggradation was driven by changes in climatic regime. Although either increased aridity or a change in precipitation regime could result in increased erosion of terrace scarps, apron aggradation events do not correlate with regional records of aridity. Instead, periods of increased precipitation intensity and/or frequency that occur during climatic transitions most likely drive apron aggradation events.

Global comparison of magnetospheric ion fluxes and auroral precipitation during a substorm

Integrated fluxes from global images taken by the High Energy Neutral Atom (HENA) and the far ultraviolet (FUV) imagers on the IMAGE spacecraft were compared for a six‐hour period, during which a reasonably intense substorm occurred. HENA and the FUV proton auroral imager (SI‐12) monitor emissions which are representative of trapped and precipitating magnetospheric proton fluxes, respectively. For several hours prior to substorm onset, measurements of the fluxes of lower energy (10–16 and 16–27 keV) magnetospheric Energetic Neutral Atoms (ENA‐s) by HENA and precipitating auroral protons by FUV SI‐12 show strong similarities, with the implication that, in general, proton precipitation is controlled by a steady pitch angle diffusion process. Less similarity is seen between ENA‐s and the auroral electron precipitation, which is monitored with the FUV Wideband Imaging Camera. Prior to substorm onset, ENA intensity at large radial distance (L > 8) is reduced while the overall integrated ENA flux increases signifying earthward motion and accumulation of the plasma. About 20 minutes before onset, the auroral fluxes decrease while the ENA intensity continues to grow. The observations are consistent with a pre‐onset increase in plasma pressure in the inner magnetosphere without an increase in precipitation showing more efficient trapping perhaps by the distorted nightside magnetosphere. At substorm onset the increase in precipitation intensity is very sudden while the more gradual intensification of the energetic ENA‐s continues. At onset the electron aurora shows an increase in intensity of one order of magnitude, while the increase in precipitating proton flux is only 50%. The intensification of the precipitation is relatively short lived (∼10 minutes) while the ENA substorm enhancements last about an hour.

Precipitation intensity trends in northern Italy

Recent studies on changes in precipitation intensity encompassing North America have found evidence for an increase in the relative amount of precipitation contributed by heavy and extreme rainfall events in the last 80 years. Within this context, the purpose of this paper is to verify whether such a signal can also be detected in northern Italy, where daily precipitation data are available from the beginning of the 19th century. The analysis is performed by applying the non-parametric Mann–Kendall test to mean anomaly series obtained through averaging the anomalies of some precipitation intensity statistics over five stations: Genoa (1833–1998), Milan (1858–1998), Mantova (1868–1997), Bologna (1879–1998) and Ferrara (1879–1996). It provides evidence that in northern Italy, the number of rainy days has a stronger and more significant negative trend than the corresponding precipitation amount, both on a yearly basis and in all of the seasons. As a consequence, precipitation intensity has a positive trend. The increase in precipitation intensity causes a significant positive trend in the proportion of total precipitation contributed by heavy precipitation events (i.e. daily precipitation >25 mm and daily precipitation >50 mm). The trend is mainly caused by the last 60–80 years, and is particularly evident in the periods of 1930–1945 and 1975–1995. The increase in precipitation intensity is connected to a modification of the distribution of daily precipitation values in a year, with trends that grow from the lower to the upper percentiles, and up to 4 mm/100 years for the 95th percentile. Copyright © 2000 Royal Meteorological Society

Trends in Extreme Weather and Climate Events: Issues Related to Modeling Extremes in Projections of Future Climate Change*

Projections of statistical aspects of weather and climate extremes can be derived from climate models representing possible future climate states. Some of the recent models have reproduced results previously reported in the Intergovernmental Panel on Climate Change (IPCC) Second Assessment Report, such as a greater frequency of extreme warm days and lower frequency of extreme cold days associated with a warmer mean climate, a decrease in diurnal temperature range associated with higher nighttime temperatures, increased precipitation intensity, midcontinent summer drying, decreasing daily variability of surface temperature in winter, and increasing variability of northern midlatitude summer surface temperatures. This reconfirmation of previous results gives an increased confidence in the credibility of the models, though agreement among models does not guarantee those changes will occur. New results since the IPCC Second Assessment Report indicate a possible increase of extreme heat stress events in a warm...

Studies on the infiltration-discharge of rain water and translation phenomena in soil

Infiltration and discharge of rain water and water movement in soil were investigated by a numerical analysis based on a one-dimensional infiltration model. A simple approximate formula for the piezometric head and water content distribution in soil in a steady state was obtained. Computational results in unsteady state indicated that the vertical downward speed of the wetting front is generally much larger than that of a tracer, which is assumed to be injected together with the rain water at the soil surface. Theoretical considerations were then developed to relate the movement of the wetting front and that of the tracer front to water movement in soil. Finally, numerical analyses were performed for various precipitation intensities, under various initial soil moisture conditions and with various soil properties. The speed of the wetting front approaches that of the tracer as the precipitation intensity increases, as the initial soil moisture decreases, and the grain size increases.