Variation In Amount Of Rainfall Research Articles

Evaluation of remote‐sensing‐based rainfall products through predictive capability in hydrological runoff modelling

AbstractThe emergence of regional and global satellite‐based rainfall products with high spatial and temporal resolution has opened up new large‐scale hydrological applications in data‐sparse or ungauged catchments. Particularly, distributed hydrological models can benefit from the good spatial coverage and distributed nature of satellite‐based rainfall estimates (SRFE). In this study, five SRFEs with temporal resolution of 24 h and spatial resolution between 8 and 27 km have been evaluated through their predictive capability in a distributed hydrological model of the Senegal River basin in West Africa. The main advantage of this evaluation methodology is the integration of the rainfall model input in time and space when evaluated at the sub‐catchment scale. An initial data analysis revealed significant biases in the SRFE products and large variations in rainfall amounts between SRFEs, although the spatial patterns were similar. The results showed that the Climate Prediction Center/Famine Early Warning System (CPC‐FEWS) and cold cloud duration (CCD) products, which are partly based on rain gauge data and produced specifically for the African continent, performed better in the modelling context than the global SRFEs, Climate Prediction Center MORPHing technique (CMORPH), Tropical Rainfall Measuring Mission (TRMM) and Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN). The best performing SRFE, CPC‐FEWS, produced good results with values of R2NS between 0·84 and 0·87 after bias correction and model recalibration. This was comparable to model simulations based on traditional rain gauge data. The study highlights the need for input specific calibration of hydrological models, since major differences were observed in model performances even when all SRFEs were scaled to the same mean rainfall amounts. This is mainly attributed to differences in temporal dynamics between products. Copyright © 2009 John Wiley & Sons, Ltd.

LONG-TERM DATA REVEAL COMPLEX DYNAMICS IN GRASSLAND IN RELATION TO CLIMATE AND DISTURBANCE

We conducted a long-term experiment in grassland with the aim of elucidating grassland dynamics in relation to variations in rainfall amount and spatial and temporal variation in disturbance. In particular we aimed to increase our understanding of ecosystem dynamics and function, species redundancy, invasion biology, and other related topics. We studied the dynamics of serpentine annual grassland in northern California over the period 1983-2002 in a replicated series of experimental plots comprising controls, gopher exclosures, and aboveground herbivore exclosures. Annual rainfall amount varied greatly during the study period, which included two major El Nino events and a period of prolonged below-average rainfall. Gopher disturbance was highly variable both spatially and temporally but was positively correlated with soil depth. Disturbance was reduced but not eliminated from the gopher exclosures and was significantly increased in the aboveground herbivore exclosures. Grassland dynamics were driven by rainfall amounts and distributions that had the most pronounced effects on the dominant plant species, while gopher disturbance had additional effects on the rarer species. Effects of excluding aboveground herbivores were swamped by a large increase in gopher disturbance within aboveground exclosures. Overall species numbers were reduced during a period of below-average rainfall but recovered in subsequent years. There was a large array of different responses of individual plant species to both rainfall and disturbance. Our results provide support for the insurance hypothesis, which suggests that biodiversity buffers ecosystem processes against environmental changes because different species (or phenotypes) respond differently to these changes, leading to functional compensations among species. Here, a species that was at very low abundance levels at the start of the study (Microseris douglasii) temporarily increased in abundance to become one of the dominant species in the grassland following a period of prolonged below-average rainfall. We also observed the repeated invasion of the serpentine grassland by the nonnative grass Bromus hordeaceus, which increased greatly in abundance following both of the major El Nino events. The results emphasize the importance of long-term observations in providing a context for shorter-term studies and allowing analysis of plant community responses to climate variation and disturbance, particularly in the face of ongoing global change.

Evaluating the effects of image filtering in short‐term radar rainfall forecasting for hydrological applications

AbstractRadar rainfall nowcasting at short lead times has important hydrometeorological applications in the fields of weather prediction and flood forecasting. The predictability of rainfall events can vary significantly with scale as smaller storm features are less predictable than the storm envelope motion. As a result, various techniques have been developed for filtering a radar image and deriving short‐term forecasts from the more predictable, larger storm scales. In this study, the effects of image filtering on radar nowcasting performance using the Storm Tracker Nowcasting Model (STNM) are evaluated. Radar rainfall nowcasts are evaluated for three storms exhibiting varying degrees of organisation over the Arkansas‐Red River basin. In each case, it is found that the nowcast skill decreases with the forecast lead time, increases with the verification area used around a forecast location, and decreases with higher rainfall thresholds. Furthermore, it is demonstrated that a set of properly tuned filtering nowcasts are superior to simple ‘persistence’ and slightly better than ‘uniform advection’. At the scale of a large hydrologic basin (∼6000 km2), filter‐based nowcasting is shown to capture the temporal variation in rainfall amount and its spatial distribution based on a set of catchment‐based metrics. Finally, a method for relating changes in nowcasting skill to errors associated with storm dynamics not captured by image filtering techniques is evaluated. Copyright © 2006 John Wiley & Sons, Ltd.

An integrated assessment of long-term changes in the hydrology of three lowland rivers in eastern England

The flow records of the Rivers Bure, Nar and Wensum in eastern England have been examined with the aim of identifying long-term changes in flow behaviour relating to variations in rainfall amount, land use, land drainage intensity and water resources use. In the study area, and since 1931, there is no evidence of long-term change in rainfall amount or distribution, on either an annual or seasonal basis. Despite changes in water resources use and catchment characteristics since the beginning of the century, such as the ending of water milling and increased land drainage and arable farming, rainfall-runoff modelling over the period 1964–1992 showed that the relationship between rainfall and runoff has remained essentially unchanged in the three study rivers. A catchment resource model used to ‘naturalise’ the historic flow records for the period 1971–1992 to account for the net effect of water supply abstractions and discharges revealed that mean river flows have been altered by surface water and groundwater abstractions, although the average losses to mean weekly flows due to net abstractions for all water uses was no greater than 3%. Greater losses occurred during drought periods as a result of increased consumptive use of water for spray irrigation and amounted to a maximum loss of 24% in the Nar catchment. In lowland areas such as eastern England that are prone to summer dry weather and periodic drought conditions, an integrated approach to river basin management, as advocated by the EU Framework Directive, is recommended for future management of surface and groundwater resources for public water supplies, river regulation purposes and industrial and agricultural demands.

A rainfall simulation model for agricultural development in Bangladesh

A rainfall simulation model based on a first‐order Markov chain has been developed to simulate the annual variation in rainfall amount that is observed in Bangladesh. The model has been tested in the Barind Tract of Bangladesh. Few significant differences were found between the actual and simulated seasonal, annual and average monthly. The distribution of number of success is asymptotic normal distribution. When actual and simulated daily rainfall data were used to drive a crop simulation model, there was no significant difference of rice yield response. The results suggest that the rainfall simulation model perform adequately for many applications.

Trade Wind Rainfall near the Windward Coast of Hawaii

A dynamically based mesoscale climatology of rainfall and other data are examined for typical trade wind conditions near Hawaii. Relative distributions of rainfall are deduced from radar reflectivity data for a 4000 km 2 region over the windward island and the upstream ocean. The cumulative rainfall data suggest three zones of forcing that are characteristic of 1) the regional ocean, 2) upstream island-induced divergence, and 3) flow reversal near and over the island. It is confirmed that most intense rainband amplifications occur over a mesoscale convergence line that separates the easterly trade winds from an island-induced westerly flow. This flow separation line resembles a classical gravity current when positioned over the windward island and near shore. The domain-scale cumulative rainfall is statistically associated with the strength of island blocking, as defined by a Froude number (Fr). When Fr is in the ordinary range (,0.3), the island maximum is five times greater than the oceanic average. When Fr is elevated (.0.3), the rainfall maximum increases to 10 times the oceanic average. Overall, the windward island maximum is seven times the upstream oceanic average. Increased rainfall is more strongly correlated with increased wind speed than with a reduction in dry static stability, both component variables of Fr. Unlike rainfall amount, the position of the island maximum is relatively insensitive to Fr, except in the hours surrounding sunrise. In the upstream divergence zone, 20‐45 km offshore, rainfall is double the oceanic background. It is speculated that the cause is blocking related. There are very strong diurnal variations in rainfall amount and distribution, both over the island and the upstream ocean. Windward island rainfall is at a maximum in the hours surrounding midnight when breeze and blocking forcings are fully cooperative in the coastal region. Seemingly unrelated, oceanic rainfall exhibits a strong nocturnal maximum, the temporal phase of which may be influenced by wind speed. A deep minimum in domain-scale rainfall occurs near noon.

Water harvesting and infiltration in arid areas affected by surface crust: examples from Jordan

In arid and semi-arid areas, surface runoff occurs at low rainfall intensities due to the presence of the surface crust. Variation in surface runoff can be attributed to the variation in rainfall amount and intensity, soil water content, infiltration, and watershed size and slope. Three micro-watersheds were selected within Muwaqar region to investigate runoff–rainfall characteristics and infiltration in soils affected by surface crust. The sites were selected to represent different sizes and slopes with well defined outlets or wadis. Flood measurements were carried out using rectangular sharp-crest contracted weirs and a Steven-water height recorder to record water height above the crest weir. In arid areas affected by surface crusts, increasing watershed size increased total volume of runoff water and decreased both volume of runoff water per unit area and runoff coefficient owing to the fact that increasing watershed size increased infiltration, evaporation and depression storage losses. Owing to reduction in infiltration, evaporation and depression storage losses as the slope of the watershed increases, increasing watershed slope increased total volume of runoff water, volume of runoff water per unit area and runoff coefficient. Rainwater harvesting works best on sloped soil with a surface crust that has a low infiltration rate. This indicates the potential for rainwater harvesting when topography and soil conditions are favourable.

Predicting watershed acidification under alternate rainfall conditions

The effect of alternate rainfall scenarios on acidification of a forested watershed subjected to chronic acidic deposition was assessed using the model of acidification of groundwater in catchments (MAGIC). The model was calibrated at the Panola Mountain Research Watershed, near Atlanta, Georgia, U.S.A. using measured soil properties, wet and dry deposition, and modeled hydrologic routing. Model forecast simulations were evaluated to compare alternate temporal averaging of rainfall inputs and variations in rainfall amount and seasonal distribution. Soil water alkalinity was predicted to decrease to substantially lower concentrations under lower rainfall compared with current or higher rainfall conditions. Soil water alkalinity was also predicted to decrease to lower levels when the majority of rainfall occurred during the growing season compared with other rainfall distributions. Changes in rainfall distribution that result in decreases in net soil water flux will temporarily delay acidification. Ultimately, however, decreased soil water flux will result in larger increases in soil-adsorbed sulfur and soil-water sulfate concentrations and decreases in alkalinity when compared to higher water flux conditions. Potential climate change resulting in significant changes in rainfall amounts, seasonal distribution of rainfall, or evapotranspiration will change net soil water flux and, consequently, will affect the dynamics of the acidification response to continued sulfate loading.

A rainfall generator for agricultural applications in the tropics

A new rainfall generator, based on a third-order Markov chain, has been developed to simulate the year-to-year variation in rainfall amount that is observed in the tropics. To do this, some of the parameters of the model are themselves sampled randomly. The model has been fitted to over 40 sites in the tropics. The model was tested in detail for three sites in Central and South America and in the African Sahel. Few significant differences were found between the historical and simulated means and variances of yearly and monthly rainfall amount and raindays per month. There were some problems with simulating wet and dry sequences of longer than 9 days. When historical and simulated rainfall records were used to drive a crop simulation model, there was no significant difference in the mean and variance of maize yield response. The results reported suggest that the rainfall model performs adequately for many applications.

Environment, Cultivar, and Ethephon Rate Interactions in Barley

AbstractEthephon[(2‐chloroethyl)phosphonic acid] reduces barley (Hordeum vulgare L.) plant height and lodging potential, but variable effects on yield components and yield also have been reported, depending on ethephon rate, cultivar, and environment. This study was conducted to investigate environment– and cultivar–by‐ethephon‐rate interactions for barley grown under Manitoba conditions. ‘Argyle’, ‘Bedford’, and ‘Samson’ barley were sown on each of two dates in 1986 and 1987, and on single dates in 1988 and 1989, at Portage la Prairie, MB on Neuhorst clay loam (fine‐loamy, frigid aquic Haploboroll). Ethephon was applied at Zadoks GS45‐47 at 0, 120, 240, 360, or 480 g ha−1. Plant height, Belgian lodging index, yield components, and yield were determined at maturity. Plant height reductions due to ethephon were unaffected by environment, and the percentage height reduction was described as a quadratic function of ethephon rate with higher increments of ethephon producing smaller increments of height reduction. Lodging only occurred in 1986, and ethephon at 240 g ha−1 eliminated lodging in most cases. Environment‐by‐cultivar‐by‐ethephon treatment interactions are significant for grain yield, spikes per square meter, and kernels per spike. Grain yield was not consistently affected by ethephon. Spikes per square meter sometimes increased, but was more often unaffected by ethephon. Kernels per spike was often reduced by ethephon. Kernel mass decreased with increasing rates of ethephon for Argyle and Bedford, but remained stable for Samson. Grain yield increased for Argyle and Bedford in cases where ethephon reduced lodging and increased spikes per square meter. Grain yield tended to decrease in cases where lodging was not a factor and ethephon reduced kernels per spike and/or kernel mass. Year‐to‐year variation in rainfall amount and distribution, and temperature may explain some of the observations made. In Manitoba, a maximum rate of 240 g ha−1 of ethephon should be used only in cases where the risk of severe lodging is high.

Influences of root distribution and growth on predicted water uptake and interspecific competition.

Root distribution and growth measured in the field were incorporated into a water uptake model for the CAM succulent Agave deserti and its nurse plant Hilaria rigida, a common desert bunchgrass. Agave deserti responds to the infrequent rainfalls of the Sonoran Desert by extending its existing established roots and by producing new roots. Most of such root growth was completed within one month after soil rewetting, total root length of A. deserti increasing by 84% for a seedling and by 58% for a mediumsized plant in the summer. Root growth in the winter with its lower soil temperatures was approximately half as much as in the summer. For a 15-year period, predicted annual root growth of A. deserti varied more than 18-fold because of annual variations in rainfall amount and pattern as well as seasonal variation in soil temperature. Predicted annual water uptake varied 47-fold over the same period. The nurse plant, which is crucial for establishment of A. deserti seedlings, reduced seedling water uptake by 38% during an average rainfall year. Lowering the location of the root system of a medium-sized A. deserti by 0.24 m reduced its simulated annual water uptake by about 25%, reflecting the importance of shallow roots for this desert succulent. Lowering the root system of a medium-sized H. rigida by 0.28 m increased the simulated annual water uptake of an associated A. deserti seedling by 17%, further indicating the influence of root overlap on competition for water.

Temporal and spatial variations in precipitation on Aldabra

Analysis of hourly rainfall data from Aldabra has indicated that the atoll influences its own weather as recorded at the Research Station on Ile Picard. The strength of this local influence varies with the season and may be related to the relative temperature difference between lagoon surface water and the atmosphere above it. The direction and strength of the prevailing wind may also be of some importance. The role of the semi-diurnal atmospheric pressure waves is not clearly evident at all times but is most prominent during the early hours of both wet and dry seasons. Its effect may also be present in the slight increase in rain frequency during the late afternoon of the trade wind seasons. The trans-atoll variations in rainfall amounts confirm the earlier reports of non-synchronous rainfalls. There is some indication of seasonal variation of rainfall at different points around the atoll relative to that received at the Research Station. The northwest and southwest parts of the atoll may receive more rain than the northeast or west central parts.

A diagram for depicting the interyearly variability in bioclimatic factors

In cases of unstable climates like that of Peshawar (West Pakistan) which exhibit variations in rainfall amount, length of distribution and season of occurrence from year to year, averages have little meaning in bioclimatic studies. Range and coefficient of variation are indicated as well as average and median values of annual precipitation, the number of rainy days in the year, the length of the dry season in months and the xerothermic index, in order to give an idea of the magnitude of variability. A diagram to illustrate the variability in the bioclimatic elements has been designed.