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1.
During this century global warming will lead to changes in global weather and climate, affecting many aspects of our environment. Agriculture is the sector of the United States economy most likely to be directly impacted by climatic changes. We have examined potential changes in dryland agriculture (Part 3) and in water resources necessary for crop production (Part 4) in response to a set of climate change scenarios. In this paper we assess to what extent, under these same scenarios, water supplies will be sufficient to meet the irrigation requirement of major grain crops in the US. In addition, we assess the overall impacts of changes in water supply on national grain production. We apply the 12 climate change scenarios described in Part 1 to the water resources and crop growth simulation models described in Part 2 for the conterminous United States. Drawing on data from Parts 3 and 4 we calculate what the aggregate national production would be in those regions in which grain crops are currently produced by applying irrigation where needed and water supplies allow. The total amount of irrigation water applied to crops declines under all climate change scenarios employed in this study. Under certain of the scenarios and in particular regions, precipitation decreases so much that water supplies are too limited; in other regions precipitation becomes so plentiful that little value is derived from irrigation. Nationwide grain crop production is greater when irrigation is applied as needed. Under irrigation, less corn and soybeans are produced under most of the climate change scenarios than is produced under baseline climate conditions. Winter wheat production under irrigation responds significantly to elevated atmospheric carbon dioxide concentrations [CO2] and appears likely to increase under climate change.  相似文献   

2.
气候变化和水的最新科学认知   总被引：5，自引：0，他引：5       下载免费PDF全文
政府间气候变化专门委员会（IPCC）于2008年4月8日正式通过了"气候变化和水"技术报告。该报告建立在IPCC 3个工作组第四次评估报告的基础上,客观、全面而审慎地评估了与水有关的气候变化以及对水的过去、现在和未来的认知。最重要的进展是：过去几十年观测到全球变暖已经与大尺度水文循环的大规模变化联系在一起;气候模型对21世纪的模拟结果一致显示出降水在高纬和部分热带地区将增加,而在部分亚热带和中低纬地区将减少的结果;预计到21世纪中期,河流年平均径流和水量可能会因为高纬和部分湿润热带地区的气候变化而增加,而在中低纬和干旱热带将可能减少;许多地方降水强度和变率的增加将使洪旱危险性上升;预计冰雪储藏的水的补给将在本世纪减少;预计较高的水温和极端变化,包括洪旱等,将影响水质并加剧水污染;对全球而言,气候变化对淡水系统负面影响将超过收益;预计由于气候变化导致的水量-水质变化将影响食物的产量、稳定性、流通和利用;气候变化影响现有水的基础设施的功能和运行,包括水电、防洪、排水、灌溉系统,同时影响到水的管理;目前的水管理措施不足以应对气候变化的影响;气候变化挑战"过去水文上的经验能得到未来的情况"的传统说法;为保障平水和干旱情况所设计的适应选择,必须综合需水和供水双方的战略;减缓措施可以降低升温对全球水资源的影响程度,进而减低适应的需求;水资源管理明显地影响到很多其他政策领域。  相似文献   

3.
The atmospheric concentration of carbon dioxide is expected to double in the next century causing increased temperatures and decreasing precipitation in some regions of the U.S. The increase in CO2 will also directly affect stomatal conductance of plants. At the first-order watershed scale, changes in evaporative demand, transpiration, and runoff will also occur. Previous modeling studies of the effect of increased CO2 on the water budgets of watersheds have been single-factor exercises where a single parameter representing stomatal conductance was reduced and the results noted. After showing validation results of the hydrology module, we used a comprehensive ecosystem model to examine the consequences of changes in precipitation, temperature, and CO2-induced plant-function characteristics on small-basin runoff. As a result of the complex interactions and of the compensatory mechanisms simulated by the model, we conclude that for arid and semiarid watersheds of the western United States, there will be little change or an actual decrease in surface runoff because of increased CO2 and climate change. This is due to the decrease in precipitation imposed on the model simulations. Implementing stomatal closure in the model did not increase runoff from the watersheds when temperatures were increased and precipitation decreased.  相似文献   

4.
The Ogallala or High Plains aquifer provides water for about 20% of the irrigated land in the United States. About 20 km3 (16.6 million acre-feet) of water are withdrawn annually from this aquifer. In general, recharge has not compensated for withdrawals since major irrigation development began in this region in the 1940s. The mining of the Ogallala has been pictured as an analogue to climate change in that many GCMs predict a warmer and drier future for this region. In this paper we attempt to anticipate the possible impacts of climate change on the sustainability of the aquifer as a source of water for irrigation and other purposes in the region. We have applied HUMUS, the Hydrologic Unit Model of the U.S. to the Missouri and Arkansas-White-Red water resource regions that overlie the Ogallala. We have imposed three general circulation model (GISS, UKTR and BMRC) projections of future climate change on this region and simulated the changes that may be induced in water yields (runoff plus lateral flow) and ground water recharge. Each GCM was applied to HUMUS at three levels of global mean temperature (GMT) to represent increasing severity of climate change (a surrogate for time). HUMUS was also run at three levels of atmospheric CO2 concentration (hereafter denoted by [CO2]) in order to estimate the impacts of direct CO2 effects on photosynthesis and evapotranspiration. Since the UKTR and GISS GCMs project increased precipitation in the Missouri basin, water yields increase there. The BMRC GCM predicts sharply decreased precipitation and, hence, reduced water yields. Precipitation reductions are even greater in the Arkansas basin under BMRC as are the consequent water yield losses. GISS and UKTR climates lead to only moderate yield losses in the Arkansas. CO2-fertilization reverses these losses and yields increase slightly. CO2 fertilization increases recharge in the base (no climate change) case in both basins. Recharge is reduced under all three GCMs and severities of climate change.  相似文献   

5.
We examined the impacts on U.S. agriculture of transient climate change assimulated by 2 global general circulation models focusing on the decades ofthe 2030s and 2090s. We examined historical shifts in the location of cropsand trends in the variability of U.S. average crop yields, finding thatnon-climatic forces have likely dominated the north and westward movement ofcrops and the trends in yield variability. For the simulated future climateswe considered impacts on crops, grazing and pasture, livestock, pesticide use,irrigation water supply and demand, and the sensitivity to international tradeassumptions, finding that the aggregate of these effects were positive for theU.S. consumer but negative, due to declining crop prices, for producers. Weexamined the effects of potential changes in El Niño/SouthernOscillation (ENSO) and impacts on yield variability of changes in mean climateconditions. Increased losses occurred with ENSO intensity and frequencyincreases that could not be completely offset even if the events could beperfectly forecasted. Effects on yield variability of changes in meantemperatures were mixed. We also considered case study interactions ofclimate, agriculture, and the environment focusing on climate effects onnutrient loading to the Chesapeake Bay and groundwater depletion of theEdward's Aquifer that provides water for municipalities and agriculture to theSan Antonio, Texas area. While only case studies, these results suggestenvironmental targets such as pumping limits and changes in farm practices tolimit nutrient run-off would need to be tightened if current environmentalgoals were to be achieved under the climate scenarios we examined  相似文献   

6.
美国全球变化研究现状   总被引：17，自引：2，他引：15  
罗勇 《气象》1999,25(1):3-8
美国的全球变化研究主要由美国全球变化研究计划（ＵＳＧＣＲＰ）支持,重点资助季节—年际尺度气候变率,十年—百年尺度的气候变化,臭氧、ＵＶ辐射以及大气化学的变化,土地利用以及陆地、海洋生态系统的变化等４个领域。当前,水汽与云仍是全球变化研究中不确定性较大的一个方面,因而受到关注。关于气候变化的信号检测以及成因分析也是一个研究热点。气候模拟研究是全球变化研究的一个主要方法。卫星资料在全球变化研究中的应用取得了大量成果。近期美国在全球变化研究领域的重点是气候模拟,短期气候预测,十年—百年尺度的气候变化,臭氧、ＵＶ辐射以及大气化学的变化,地表以及陆地、海洋生态系统变化,对全球变化的区域尺度估计,卫星资料的应用,气候变化影响的国家级评估等８个方面。  相似文献   

7.
To study the impacts of climate change on water resources in the western U.S., global climate simulations were produced using the National Center for Atmospheric Research/Department of Energy (NCAR/DOE) Parallel Climate Model (PCM). The Penn State/NCAR Mesoscale Model (MM5) was used to downscale the PCM control (20 years) and three future(2040–2060) climate simulations to yield ensemble regional climate simulations at 40 km spatial resolution for the western U.S. This paper describes the regional simulations and focuses on the hydroclimate conditions in the Columbia River Basin (CRB) and Sacramento-San Joaquin River (SSJ) Basin. Results based on global and regional simulations show that by mid-century, the average regional warming of 1 to 2.5 °C strongly affects snowpack in the western U.S. Along coastal mountains, reduction in annual snowpack was about70% as indicated by the regional simulations. Besides changes in mean temperature, precipitation, and snowpack, cold season extreme daily precipitation increased by 5 to 15 mm/day (15–20%) along theCascades and the Sierra. The warming resulted in increased rainfall at the expense of reduced snowfall, and reduced snow accumulation (or earlier snowmelt) during the cold season. In the CRB, these changes were accompanied by more frequent rain-on-snow events. Overall, they induced higher likelihood of wintertime flooding and reduced runoff and soil moisture in the summer. Changes in surface water and energy budgets in the CRB and SSJ basin were affected mainly by changes in surface temperature, which were statistically significant at the 0.95 confidence level. Changes in precipitation, while spatially incoherent, were not statistically significant except for the drying trend during summer. Because snow and runoff are highly sensitive tospatial distributions of temperature and precipitation, this study shows that (1) downscaling provides more realistic estimates of hydrologic impacts in mountainous regions such as the western U.S., and (2) despite relatively small changes in temperature and precipitation, changes in snowpack and runoff can be much larger on monthly to seasonal time scales because the effects of temperature and precipitation are integrated over time and space through various surface hydrological and land-atmosphere feedback processes. Although the results reported in this study were derived from an ensemble of regional climate simulations driven by a global climate model that displays low climate sensitivity compared with most other models, climate change was found to significantly affect water resources in the western U.S. by the mid twenty-first century.  相似文献   

8.
Climate change impacts on the regional hydrological cycle are compared for model projections following an ambitious emissions-reduction scenario (E1) and a medium-high emissions scenario with no mitigation policy (A1B). The E1 scenario is designed to limit global annual mean warming to 2 °C or less above pre-industrial levels. A multi-model ensemble consisting of ten coupled atmosphere–ocean general circulation models is analyzed, which includes five Earth System Models containing interactive carbon cycles. The aim of the study is to assess the changes that could be mitigated under the E1 scenario and to identify regions where even small climate change may lead to strong changes in precipitation, cloud cover and evapotranspiration. In these regions the hydrological cycle is considered particularly vulnerable to climate change, highlighting the need for adaptation measures even if strong mitigation of climate change would be achieved. In the A1B projections, there are significant drying trends in sub-tropical regions, precipitation increases in high latitudes and some monsoon regions, as well as changes in cloudiness and evapotranspiration. These signals are reduced in E1 scenario projections. However, even under the E1 scenario, significant precipitation decrease in the subtropics and increase in high latitudes are projected. Particularly the Amazon region shows strong drying tendencies in some models, most probably related to vegetation interaction. Where climate change is relatively small, the E1 scenario tends to keep the average magnitude of potential changes at a level comparable to current intra-seasonal to inter-annual variability at that location. Such regions are mainly located in the mid-latitudes.  相似文献   

9.
Here we simulate dryland agriculture in the United States in order to assess potential future agricultural production under a set of general circulation model (GCM)-based climate change scenarios. The total national production of three major grain crops—corn, soybeans, and winter wheat—and two forage crops—alfalfa and clover hay—is calculated for the actual present day core production area (CPA) of each of these crops. In general, higher global mean temperature (GMT) reduces production and higher atmospheric carbon dioxide concentration ([CO2]) increases production. Depending on the climatic change scenarios employed overall national production of the crops studied changes by up to plus or minus 25% from present-day levels. Impacts are more significant regionally, with crop production varying by greater than ±50% from baseline levels. Analysis of currently possible production areas (CPPAs) for each crop indicates that the regions most likely to be affected by climate change are those on the margins of the areas in which they are currently grown. Crop yield variability was found to be primarily influenced by local weather and geographic features rather than by large-scale changes in climate patterns and atmospheric composition. Future US agronomic potential will be significantly affected by the changes in climate projected here. The nature of the crop response will depend primarily on to what extent precipitation patterns change and also on the degree of warming experienced.  相似文献   

10.
Tropical land cover change experiments with fixed sea-surface temperatures (SSTs) and with an interactive ocean are compared to assess the relevance of including the ocean system in sensitivity studies to land surface conditions. The results show that the local response to deforestation is similar with fixed and simulated SSTs. Over Amazonia, all experiments simulate a comparable decrease in precipitation and no change in moisture convergence, implying that there is only a change in local water recycling. Over Africa, the impact on precipitation is not identical for all experiments; however, the signal is smaller than over Amazonia and simulations of more than 50 years would be necessary to statistically discriminate the precipitation change. We observe small but significant changes in SSTs in the coupled simulation in the tropical oceans surrounding the deforested regions. Impacts on mid and high latitudes SSTs are also possible. As remote impacts to deforestation are weak, it has not been possible to establish possible oceanic feedbacks to the atmosphere. Overall, this study indicates that the oceanic feedback to land surface sensitivity studies is of second importance, and that the inclusion of the oceanic system will require ensembles of long climate simulations to properly take into account the low frequency variability of the ocean.  相似文献   

11.
Cultural ecosystem services represent nonmaterial benefits people derive from the environment; these benefits include outdoor recreation opportunities. Changes in climatic conditions are likely to shift the spatial and temporal demand for recreational ecosystem services. To date, little is known about the magnitude and spatial variability in these shifts across large geographic extents. We use 14 years of geotagged social media data to explore how the climatological mean of maximum temperature affects the demand for recreational ecosystem services by season across public lands in the continental United States. We also investigate how the demand for recreational ecosystem services on public lands may change by 2050 under two climate change scenarios, RCP 4.5 and RCP 8.5. Across all public lands in the continental U.S., demand for recreational ecosystem services is expected to decrease 18% by 2050 under RCP 4.5 in the summer, but increase 12% in the winter and 5% in the spring, with no significant changes in the fall. There is substantial variation in the magnitude of projected changes by region. In the spring and fall, some regions are likely to see an increase in the demand for recreational ecosystem services (e.g., Arkansas-Rio Grande-Texas-Gulf), while others will see declines (e.g., South Atlantic Gulf, California Great Basin). Our findings suggest the total demand for recreational ecosystem services across the continental U.S. is expected to decline under warming temperatures. However, there is a large amount of variation in where, when, and by how much, demand will change. The peak season for visiting public lands is likely to lengthen in the continental U.S. as the climate continues to warm, with demand declining in the summer and growing in the off-season.  相似文献   

12.
Hydrological models of the Great Lakes basin were used to study the sensitivity of Great Lakes water supplies to climate warming by driving them with meteorological data from four U.S. climate zones that were transposed to the basin. Widely different existing climates were selected for transposition in order to identify thresholds of change where major impacts on water supplies begin to occur and whether there are non-linear responses in the system. The climate zones each consist of 43 years of daily temperature and precipitation data for 1,000 or more stations and daily evaporation-related variables (temperature, wind speed, humidity, cloud cover) for approximately 20–35 stations. A key characteristic of these selected climates was much larger variability in inter-annual precipitation than currently experienced over the Great Lakes. Climate data were adjusted to simulate lake effects; however, a comparison of hydrologic results with and without lake effects showed that there was only minor effects on water supplies.  相似文献   

13.
This study uses empirical agricultural impact models to compare the U.S. climate change predictions of 16 General Circulation Models (GCMs). The impact analysis provides a policy-relevant index by which to judge complex climate predictions. National aggregate impacts vary widely across the 16 GCMs because of varying regional and seasonal patterns of predicted climate change. Examining the predicted impacts from the full set of GCMs reveals that the seasonal detail in the GCM predictions is so noisy that it is not significantly different from a constant annual change. However, a consistent regional pattern does emerge across the set of models. Nonetheless, aggregating climate change across seasons and regions within the United States, using a national-annual climate change provides a reasonable and efficient approximation to the expected impact predicted by the 16 GCM models.  相似文献   

14.
The likely intensification of extreme droughts from climate change in many regions across the United States has increased interest amongst researchers and water managers to understand not only the magnitude of drought impacts and their consequences on water resources, but also what they can do to prevent, respond to, and adapt to these impacts. Building and mobilizing ‘adaptive capacity’ can help in this pursuit. Researchers anticipate that drought preparedness measures will increase adaptive capacity, but there has been minimal testing of this and other assumptions about the governance and institutional determinants of adaptive capacity. This paper draws from recent extreme droughts in Arizona and Georgia to empirically assess adaptive capacity across spatial and temporal scales. It combines quantitative and qualitative methodologies to identify a handful of heuristics for increasing adaptive capacity of water management to extreme droughts and climate change, and also highlights potential tradeoffs in building and mobilizing adaptive capacity across space and time.  相似文献   

15.
Impact of ENSO-Related Climate Anomalies on Crop Yields in the U.S.   总被引：1，自引：1，他引：0  
Historical daily thermal and precipitation data from selected stations across the United States are composited into climate scenarios for the three phases of ENSO: Warm Events (El Niño), Cold Events (El Viejo or La Niña), and Neutral. Using these scenarios, yields of 7 field crops were simulated using the EPIC biophysical model during the one-year period coincident with maximum SST anomalies in the equatorial Pacific. The response of simulated agricultural productivity to the ENSO-related climate-variability parameters, is presented. A sensitivity calculation confirms the relevance of precipitation totals/medians and suggests ENSO-related yields are sensitive to changes in statistical properties characterizing precipitation distribution and occurrence. Results are spatially dependent, with the southwest and northern plains regions indicating the highest sensitivity to the inclusion of additional precipitation characteristics. The southeast yields are not as sensitive. The yield deviations (expressed as normalized differences to neutral yields) associated with the two extreme ENSO phases (Warm Events and Cold Events) are spatially and crop dependent with ranges up to ±120%. The largest yield deviations are in the south, southwest, and northern plains. Overall, Cold Events demonstrate larger impacts in the southern regions and Warm Events have a larger impact in the north. Additionally, the notion that climate anomalies associated with Cold and Warm Events and subsequent impacts on yields should be of opposite sign (i.e., linear) is not valid in many regions. For the eastern half of the U.S., modeled yield deviations under Warm Event conditions are nearly all less than neutral. Conversely, in the western half, results are more mixed. Under Cold Event conditions, yields in the east are enhanced in the south, but worsened in the north; while in the western half, yields have decreased in general. The results highlight the critical role of climate and production-related data on station or county levels in quantifying the impact of ENSO climate anomalies on yields. Both the diverse nature of the ENSO-related yield deviations as well as their sensitivity to monthly frequency distribution and occurrence characteristics imply that ENSO-related seasonal precipitation forecasts might be beneficial for agricultural application only if details were provided regarding not only totals, but also predicted changes in temporal and spatial variability of a more comprehensive suite of characteristics.  相似文献   

16.
Effects of climate on numbers of northern prairie wetlands   总被引：4，自引：1，他引：4  
The amount of water held in individual wetland basins depends not only on local climate patterns but also on groundwater flow regime, soil permeability, and basin size. Most wetland basins in the northern prairies hold water in some years and are dry in others. To assess the potential effect of climate change on the number of wetland basins holding water in a given year, one must first determine how much of the variability in number of wet basins is accounted for by climatic variables. I used multiple linear regression to examine the relationship between climate variables and percentage of wet basins throughout the Prairie Pothole Region of Canada and the United States. The region was divided into three areas: parkland, Canadian grassland, and United States grassland (i.e., North Dakota and South Dakota). The models - which included variables for spring and fall temperature, yearly precipitation, the previous year's count of wet basins, and for grassland areas, the previous fall precipitation - accounted for 63 to 65% of the variation in the number of wet basins. I then explored the sensitivities of the models to changes in temperature and precipitation, as might be associated with increased greenhouse gas concentrations. Parkland wetlands are shown to be much more vulnerable to increased temperatures than are wetlands in either Canadian or United States grasslands. Sensitivity to increased precipitation did not vary geographically. These results have implications for waterfowl and other wildlife populations that depend on availability of wetlands in the parklands for breeding or during periods of drought in the southern grasslands.The U.S. Government right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.  相似文献   

17.
Climate Change and Water Resources   总被引：13，自引：1，他引：13  
Current perspectives on global climate change based on recent reports of the Intergovernmental Panel on Climate Change (IPCC) are presented. Impacts of a greenhouse warming that are likely to affect water planning and evaluation include changes in precipitation and runoff patterns, sea level rise, land use and population shifts following from these effects, and changes in water demands. Irrigation water demands are particularly sensitive to changes in precipitation, temperature, and carbon dioxide levels. Despite recent advances in climate change science, great uncertainty remains as to how and when climate will change and how these changes will affect the supply and demand for water at the river basin and watershed levels, which are of most interest to planners. To place the climate-induced uncertainties in perspective, the influence on the supply and demand for water of non-climate factors such as population, technology, economic conditions, social and political factors, and the values society places on alternative water uses are considered.  相似文献   

18.
Human activities have altered the distribution and quality of terrestrial ecosystems. Future demands for goods and services from terrestrial ecosystems will occur in a world experiencing human-induced climate change. In this study, we characterize the range in response of unmanaged ecosystems in the conterminous U.S. to 12 climate change scenarios. We obtained this response by simulating the climatically induced shifts in net primary productivity and geographical distribution of major biomes in the conterminous U.S. with the BIOME 3 model. BIOME 3 captured well the potential distribution of major biomes across the U.S. under baseline (current) climate. BIOME 3 also reproduced the general trends of observed net primary production (NPP) acceptably. The NPP projections were reasonable for forests, but not for grasslands where the simulated values were always greater than those observed. Changes in NPP would be most severe under the BMRC climate change scenario in which severe changes in regional temperatures are projected. Under the UIUC and UIUC + Sulfate scenarios, NPP generally increases, especially in the West where increases in precipitation are projected to be greatest. A CO2-fertilization effect either amplified increases or alleviated losses in modeled NPP. Changes in NPP were also associated with changes in the geographic distribution of major biomes. Temperate/boreal mixed forests would cover less land in the U.S. under most of the climate change scenarios examined. Conversely, the temperate conifer and temperate deciduous forests would increase in areal extent under the UIUC and UIUC + Sulfate scenarios. The Arid Shrubland/Steppe would spread significantly across the southwest U.S. under the BMRC scenario. A map overlay of the simulated regions that would lose or gain capacity to produce corn and wheat on top of the projected distribution of natural ecosystems under the BMRC and UIUC scenarios (Global mean temperature increase of +2.5 °C, no CO2 effect) helped identify areas where natural and managed ecosystems could contract or expand. The methods and models employed here are useful in identifying; (a) the range in response of unmanaged ecosystem in the U.S. to climate change and (b) the areas of the country where, for a particular scenario of climate change, land cover changes would be most likely.  相似文献   

19.
Jinwon Kim 《Climatic change》2005,68(1-2):153-168
The effects of increased atmospheric CO2 on the frequency of extreme hydrologic events in the Western United States (WUS) for the 10-yr period of 2040–2049 are examined using dynamically downscaled regional climate change signals. For assessing the changes in the occurrence of hydrologic extremes, downscaled climate change signals in daily precipitation and runoff that are likely to indicate the occurrence of extreme events are examined. Downscaled climate change signals in the selected indicators suggest that the global warming induced by increased CO2 is likely to increase extreme hydrologic events in the WUS. The indicators for heavy precipitation events show largest increases in the mountainous regions of the northern California Coastal Range and the Sierra Nevada. Increased cold season precipitation and increased rainfall-portion of precipitation at the expense of snowfall in the projected warmer climate result in large increases in high runoff events in the Sierra Nevada river basins that are already prone to cold season flooding in todays climate. The projected changes in the hydrologic characteristics in the WUS are mainly associated with higher freezing levels in the warmer climate and increases in the cold season water vapor influx from the Pacific Ocean.  相似文献   

20.
As carbon dioxide and other greenhouse gasses accumulate in the atmosphere and contribute to rising global temperatures, it is important to examine how a changing climate may affect natural and managed ecosystems. In this series of papers, we study the impacts of climate change on agriculture, water resources and natural ecosystems in the General Circulation Model (GCM)-derived climate change projections, described in Part 1, to drive the crop production and water resource models EPIC (Erosion Productivity Impact Calculator) and HUMUS (Hydrologic Unit Model of the United States). These models are described and validated in this paper using historical crop yields and streamflow data in the conterminous United States in order to establish their ability to accurately simulate historical crop and water conditions and their capability to simulate crop and water response to the extreme climate conditions predicted by GCMs. EPIC simulated grain and forage crop yields are compared with historical crop yields from the US Department of Agriculture (USDA) and with yields from agricultural experiments. EPIC crop yields correspond more closely with USDA historical county yields than with the higher yields from intensively managed agricultural experiments. The HUMUS model was validated by comparing the simulated water yield from each hydrologic basin with estimates of natural streamflow made by the US Geological Survey. This comparison shows that the model is able to reproduce significant observed relationships and capture major trends in water resources timing and distribution across the country.  相似文献   

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