An innovative approach for locating and evaluating subsurface pathways for nitrogen loss

Fundamental watershed-scale processes governing chemical flux to neighboring ecosystems are so poorly understood that effective strategies for mitigating chemical contamination cannot be formulated. Characterization of evapotranspiration, surface runoff, plant uptake, subsurface preferential flow, behavior of the chemicals in neighboring ecosystems, and an understanding of how crop management practices influence these processes are needed. Adequate characterization of subsurface flow has been especially difficult because conventional sampling methods are ineffective for measuring preferential flow of water and solutes. A sampling strategy based on ground-penetrating radar (GPR) mapping of subsurface structures coupled with near real-time soil moisture data, surface topography, remotely sensed imagery, and a geographic information system (GIS) appears to offer a means of accurately identifying subsurface preferential flow pathways. Four small adjacent watersheds draining into a riparian wetland and first-order stream at the USDA-ARS Beltsville Agricultural Research Center, Beltsville, MD are being studied with this protocol. The spatial location of some preferential flow pathways for chemicals exiting these agricultural watersheds to the neighboring ecosystems have been identified. Confirmation of the pathways is via examination of patterns in yield monitor data and remote sensing imagery.     

流域水质管理系统构建的理论、方法和实践

随着工农业的发展及乡村都市化 ,淡水资源的短缺成为全球性的问题。淡水资源的短缺 ,一方面是对淡水需求量增加 ,供不应求。另一方面是水体水质恶化 ,水资源退化。保护淡水资源是一项持久性的艰巨任务 ,其中水资源管理工具评价预测各种管理措施对水资源的影响 ,是必不可缺少的。工业和生活废水对水体的点源污染问题 ,早在2 0世纪 2 0年代就被意识到 ,并开展了一些水资源的保护和管理研究工作。自 192 5第一个水质数学模型Ｓｔｒｅｅｔｅｒ Ｐｈｅｌｐｓ[1] 用于模拟水环境中ＢＯＤ和ＤＯ的动态变化研究以来 ,出现了许多水质模型并用于河流、…     

Surface runoff contribution of nitrogen during storm events in a forested watershed

We examined total Kjeldahl nitrogen (TKN) loading to a small forested stream during storm events. We hypothesized that upper soil and litter layers in riparian area are primary source of higher TKN concentrations during storm. A storm water sampling program was carried out to gather requisite flow and water quality data to calibrate and validate water and nutrient components of the Riparian Ecosystem Management Model for TKN. Water quality and storm flow data collected from January 2000 to December 2003 were used to simulate the hydrology and nitrogen transport over a second-order watershed within the Fort Benning Military Installation, Georgia. Intensive sampling conducted from October 2002 to May 2003 provided the necessary data to characterize the rising limb, peak, and recession limb of six major storm events. Simulated runoff and storm TKN loads were compared with their corresponding observed or calculated values. Hydrology and nitrogen data collected from February 21, 2003 to December 31, 2003 were used for the model validation. The hydrology component of the model showed a Nash-Sutcliffe efficiency of 87% for the validation period. The average absolute difference between simulated and calculated TKN loads was 25%. Even though the monthly water budget indicated the dominance of subsurface flow, TKN contribution from direct runoff was significantly greater than that from subsurface flow. On an average, 73% of the observed total TKN load at the watershed outlet was contributed by surface runoff during storm events. The results suggested that the surface runoff during the storm events washed off the nitrogen from the forest floor and transported to the stream.     

基于SIMWE模型的典型水土保持措施侵蚀阻控路径分析——以通双小流域为例

针对东北黑土区长缓坡地形条件下坡面产汇流集中易加剧土壤侵蚀的问题,本研究基于GIS和SIMWE(SIMulated Water Erosion)模型,引入连通性指数和水深空间分布作为水文连通性与径流路径的衡量指标.通过量化不同典型水土保持措施对土壤入渗速率和地表曼宁糙率的影响,构建梯田数字高程模型(DEM)模拟表征地表...     

模拟降雨下喀斯特坡耕地土壤养分输出机制

喀斯特区坡耕地水土及养分流失不仅是该区土地质量退化、土地生产力衰退主要原因,同时也是该区地下水质污染的重要因素。为揭示喀斯特坡耕地地表和地下二元空间结构下的土壤养分流失机制,以喀斯特坡耕地为研究对象,通过模拟其地表微地貌及地下孔(裂)隙构造特征,采用人工模拟降雨试验研究不同雨强下喀斯特坡耕地地表及地下水土及其氮、磷、钾流失特征。结果表明:(1)小雨强(50mm/h)和中雨强下(70mm/h),喀斯特坡耕地坡面产流主要以地下产流为主;大雨强下(90mm/h),地表径流高于地下径流;产沙方式则表现为由小雨的地表和地下产沙并重到中大雨强的地表产沙为主的一个转变过程。(2)在降雨侵蚀过程中,径流各养分输出浓度均表现出一定的初期冲刷效应,受土壤吸附作用影响,雨强对全钾(TK)和全氮(TN)的影响较全磷(TP)明显。(3)地表径流、地表泥沙和总泥沙各养分输出负荷均随雨强增大而增加,坡面径流泥沙总的TK输出负荷以泥沙为主,而TN和TP输出负荷则以径流为主;TP和TN在径流的输出负荷上以地下径流输出为主(其中TP地表负荷比在11.6%—46.2%,TN在7.0%—48.5%之间),而TK则以二者并重(地表负荷比在43.5%—57.0%之间);各养分在泥沙的输出负荷上则均以地表泥沙流失为主,其负荷比均在54.5%以上。研究结果可为喀斯特区坡耕地水土流失及养分流失的源头控制提供基本参数和科学依据。     

Regional comparison of nitrogen export to Japanese forest streams

Nitrogen (N) emissions in Asian countries are predicted to increase over the next several decades. An understanding of the mechanisms that control temporal and spatial fluctuation of N export to forest streams is important not only to quantify critical loads of N, N saturation status, and soil acidification N dynamics and budgets in Japanese forested watersheds is not clear due to the lack of regional comparative studies on stream N chemistry. To address the lack of comparative studies, we measured inorganic N (nitrate and ammonium) concentrations from June 2000 to May 2001 in streams in 18 experimental forests located throughout the Japanese archipelago and belonging to the Japanese Union of University Forests. N concentrations in stream water during base flow and high flow periods were monitored, and N mineralization potential in soil was measured using batch incubation experiments. Higher nitrate concentrations in stream water were present in central Japan, an area that receives high rates of atmospheric N deposition. In northern Japan, snowmelt resulted in increased nitrate concentrations in stream water. The potential net N mineralization rate was higher in surface soil than in subsurface soil, and the high potential for N mineralization in the surface soil partly contributed to the increase in nitrate concentration in stream water during a storm event. Regional differences in the atmospheric N deposition and seasonality of precipitation and high discharge are principal controls on the concentrations and variations of nitrates in stream water in forested watersheds of Japan.     

Runoff sources and land cover change in the Amazon: an end-member mixing analysis from small watersheds

The flowpaths by which water moves from watersheds to streams has important consequences for the runoff dynamics and biogeochemistry of surface waters in the Amazon Basin. The clearing of Amazon forest to cattle pasture has the potential to change runoff sources to streams by shifting runoff to more surficial flow pathways. We applied end-member mixing analysis (EMMA) to 10 small watersheds throughout the Amazon in which solute composition of streamwater and groundwater, overland flow, soil solution, throughfall and rainwater were measured, largely as part of the Large-Scale Biosphere-Atmosphere Experiment in Amazonia. We found a range in the extent to which streamwater samples fell within the mixing space determined by potential flowpath end-members, suggesting that some water sources to streams were not sampled. The contribution of overland flow as a source of stream flow was greater in pasture watersheds than in forest watersheds of comparable size. Increases in overland flow contribution to pasture streams ranged in some cases from 0% in forest to 27?C28% in pasture and were broadly consistent with results from hydrometric sampling of Amazon forest and pasture watersheds that indicate 17- to 18-fold increase in the overland flow contribution to stream flow in pastures. In forest, overland flow was an important contribution to stream flow (45?C57%) in ephemeral streams where flows were dominated by stormflow. Overland flow contribution to stream flow decreased in importance with increasing watershed area, from 21 to 57% in forest and 60?C89% in pasture watersheds of less than 10?ha to 0% in forest and 27?C28% in pastures in watersheds greater than 100?ha. Soil solution contributions to stream flow were similar across watershed area and groundwater inputs generally increased in proportion to decreases in overland flow. Application of EMMA across multiple watersheds indicated patterns across gradients of stream size and land cover that were consistent with patterns determined by detailed hydrometric sampling.     

Modeling the Impact of Soil and Water Conservation on Surface and Ground Water Based on the SCS and Visual Modflow

Soil and water conservation measures can impact hydrological cycle, but quantitative analysis of this impact is still difficult in a watershed scale. To assess the effect quantitatively, a three-dimensional finite-difference groundwater flow model (MODFLOW) with a surface runoff model–the Soil Conservation Service (SCS) were calibrated and applied based on the artificial rainfall experiments. Then, three soil and water conservation scenarios were simulated on the sand-box model to assess the effect of bare slope changing to grass land and straw mulching on water volume, hydraulic head, runoff process of groundwater and surface water. Under the 120 mm rainfall, 60 mm/h rainfall intensity, 5 m² area, 3° slope conditions, the comparative results indicated that the trend was decrease in surface runoff and increase in subsurface runoff coincided with the land-use converted from bare slope to grass land and straw mulching. The simulated mean surface runoff modulus was 3.64×10⁻² m³/m²/h in the bare slope scenario, while the observed values were 1.54×10⁻² m³/m²/h and 0.12×10⁻² m³/m²/h in the lawn and straw mulching scenarios respectively. Compared to the bare slope, the benefits of surface water reduction were 57.8% and 92.4% correspondingly. At the end of simulation period (T = 396 min), the simulated mean groundwater runoff modulus was 2.82×10⁻² m³/m²/h in the bare slope scenario, while the observed volumes were 3.46×10⁻² m³/m²/h and 4.91×10⁻² m³/m²/h in the lawn and straw mulching scenarios respectively. So the benefits of groundwater increase were 22.7% and 60.4% correspondingly. It was concluded that the soil and water conservation played an important role in weakening the surface runoff and strengthening the underground runoff. Meanwhile the quantitative analysis using a modeling approach could provide a thought for the study in a watershed scale to help decision-makers manage water resources.     

模拟降雨下植被盖度对坡面流水动力学特性的影响

通过模拟降雨实验的方法,分析研究了坡度10°和20°,降雨强度30mm/h和60mm/h条件下不同盖度黑麦草对坡面产流产沙的调控过程,并从雷诺数、弗劳德数和阻力系数三个方面对水流运动过程和黑麦草调控坡面流的水力学特性进行了剖析。研究结果表明:雷诺数随坡度增加而相对增大,随降雨强度增大有明显增大趋势。黑麦草覆盖能够明显减小坡面径流雷诺数,在各降雨强度和坡度条件下,雷诺数随黑麦草盖度增加而减小,雷诺数大小一般呈现:裸坡20%40%60%80%。黑麦草盖度对坡面流弗劳德数有显著影响,随着黑麦草盖度增加弗劳德数减小,并且弗劳德数随盖度变化为:裸地20%40%60%80%,坡面阻力系数与坡面产沙率有良好的拟合关系,随坡面阻力系数的增大,坡面产沙率呈对数减小,并且阻力系数在0—1时减小速率很大,阻力系数大于1以后减小曲线较为平缓。     

小流域林草植被控制土壤侵蚀机理研究

从坡面水动力学角度研究了坡面乔木林、草本植物和林地枯落物对坡面径流流速和动能的影响机理.结果表明,坡面径流水头损失与坡面坡度、林木密度、净雨强、坡长等有关,水头损失与林木间距(b)和树木地径(D)间的关系为:E^∝(D/b)^4/3;坡面草本植物在水流作用下易弯曲,增大水流底层的阻力,减小床面的切应力;枯枝落叶使径流速度减小,从而大大降低径流挟沙能力.对甘肃省天水市桥子东沟和桥子西沟两个对比小流域的实测单次降雨、径流、泥沙资料分析可见,在相同降水条件下,已治理小流域内的径流量、产沙量、洪峰流量、最大输沙率等指标均小于未治理小流域,说明林草植被在小流域中的涵养水源、保持水土的作用明显.     

Groundwater nitrate removal in riparian buffer zones: a review of research progress in the past 20 years

This review evaluates research in the past 20 years focusing on groundwater nitrate removal in the riparian zones of agricultural watersheds. Studies have reported a large range in the magnitude of groundwater and nitrate fluxes to buffers in different hydrogeologic settings. An earlier focus on buffers with shallow subsurface flow has expanded to include sites with deep flow paths and groundwater-fed overland flow. Nitrate removal efficiency and the width required for removal have been linked to riparian sediment texture and depth to an impervious layer. Denitrification has been identified as the dominant mechanism of nitrate removal based on evidence that this process occurs at depth in many buffers which contain buried organic-rich deposits. Several studies have assessed the cumulative effect of riparian buffers on nitrate removal at the watershed scale. Despite considerable research progress areas of uncertainty still remain. Buffers with coarse-textured sediments located in landscapes with upslope sand aquifers have received most attention. In contrast, few sites have been analysed in weathered bedrock and glacial till landscapes. Many studies have reported nitrate removal efficiency based on nitrate concentrations rather than measuring groundwater fluxes which assess the magnitude of nitrate removal. More information is needed on interactions between riparian hydrological flow paths and biogeochemical processes. Further research is recommended on the effect of riparian zone nitrate removal at the watershed scale and long-term monitoring with respect to buffer restoration, the ability to sustain nitrate removal and responses to land use and climate change.

     

中梁山岩溶槽谷区不同土地利用方式坡地产流规律

坡地产流是造成岩溶区水土流失的主要驱动力,研究典型岩溶槽谷区坡地产流规律,对岩溶区防治水土流失、合理利用地下水资源具有重要理论意义。在重庆市中梁山龙凤和龙车槽谷选取不同土地利用方式的4个标准径流小区,对降水、地表径流、壤中流、裂隙流和土壤含水率进行了同步监测,探讨了坡地产流特征。结果表明:(1)4个不同土地利用方式的径流小区,坡地总产流量从大到小依次为:耕地(3696.9L)果园地(3657.2L)竹林地(2922.9L)林地(2211.1L),总径流系数(3.1%—5.2%)远低于非岩溶区(约20%);(2)4个径流小区的产流形式主要为地表径流,壤中流和裂隙流产生滞后于地表径流;(3)降水因子、前期土壤含水率共同影响地表径流,但降水因子对地表径流的影响远大于前期土壤含水率。降水因子中,15min最大雨强是影响耕地、果园地的地表径流的主要因素,降水量是影响林地、竹林地的地表径流的主要因素;前期土壤含水率对耕地、林地、果园地地表径流影响较大,对竹林地地表径流影响较小。     

干热河谷区不同坡位土壤水分对降雨的响应特征

研究干热河谷地区土壤水分在降雨过程中的短时动态变化,有助于揭示该地区的土壤水文功能。本研究选取贵州花江干热河谷作为研究区,运用原位监测法,获取不同坡位的高频土壤水分监测数据,分析土壤水分对降雨的短时动态响应特征。结果表明: 在整个监测期间,无论是坡上还是坡中,研究区各层土壤水分均为中等变异水平(15.2%≤变异系数CV≤29.7%),坡上土壤水分的波动幅度(CV=21.1%)大于坡中(CV=19.1%),0～5 cm土层(CV=26.2%)大于20～40 cm土层(CV=16.5%)。与坡中相比,坡上土壤水分对降雨的响应速度更快,降雨对土壤水分的补给量大、补给速率快;坡上的土壤水分补给速率与消退速率之差(2.3%·h^-1)大于坡中(1.8%·h^-1)。随土层深度增加,下层土壤水分对降雨的响应早于或同步于上层,降雨对土壤水分的补给量减少、补给速率减慢,土壤水分的消退速率也减慢。与坡中相比,坡上土壤水分入渗能力更强,保水能力更优。干热河谷的微观环境和小气候影响土壤水分对降雨的响应特征,而岩-土界面优先流的快速补给则会加快下层土壤水分对降雨的响应速度,使得该地区的坡面更容易形成混合产流机制。     

The influence of tidal marshes on upland groundwater discharge to estuaries

We investigated subsurface hydrology in two fringing tidal marshes and in underlying aquifers in the coastal plain of Virginia. Vertical distributions of hydraulic conductivity, hydraulic head and salinity were measured in each marsh and a nearby subtidal sediment. Discharge of hillslope groundwater into the base of the marshes and subtidal sediment was calculated using Darcy's law. In the marshes, fluxes of pore water across the sediment surface were measured or estimated by water balance methods. The vertical distribution of salt in shoreline sediments was modeled to assess transport and mixing conditions at depth. Hydraulic gradients were upward beneath shoreline sediments; indicating that groundwater was passing through marsh and subtidal deposits before reaching the estuary. Calculated discharge (6 to 10 liters per meter of shoreline per day) was small relative to fluxes of pore water across the marsh surface at those sites; even where discharge was maximal (at the upland border) it was 10 to 50 times less than infiltration into marsh soils. Pore water turnover in our marshes was therefore dominated by exchange with estuarine surface water. In contrast, new interstitial water entering subtidal sediments appeared to be primarily groundwater, discharged from below. The presence of fringing tidal marshes delayed transport and increased mixing of groundwater and solute as it traveled towards the estuaries. Soil-contact times of discharged groundwater were up to 100% longer in marshes than in subtidal shoreline sediments. Measured and modeled salinity profiles indicated that, prior to export to estuaries, the solutes of groundwater, marsh pore water and estuarine surface water were more thoroughly mixed in marsh soils compared to subtidal shoreline sediments. These findings suggest that transport of reactive solutes in groundwater may be strongly influenced by shoreline type. Longer soil-contact times in marshes provide greater opportunity for immobilization of excess nutrients by plants, microbes and by adsorption on sediment. Also, the greater dispersive mixing of groundwater and pore water in marshes should lead to increased availability of labile, dissolved organic carbon at depth which could in turn enhance microbial activity and increase the rate of denitrification in situations where groundwater nitrate is high.     

The influence of tidal marshes on upland groundwater discharge to estuaries

We investigated subsurface hydrology in two fringing tidal marshes and in underlying aquifers in the coastal plain of Virginia. Vertical distributions of hydraulic conductivity, hydraulic head and salinity were measured in each marsh and a nearby subtidal sediment. Discharge of hillslope groundwater into the base of the marshes and subtidal sediment was calculated using Darcy's law. In the marshes, fluxes of pore water across the sediment surface were measured or estimated by water balance methods. The vertical distribution of salt in shoreline sediments was modeled to assess transport and mixing conditions at depth. Hydraulic gradients were upward beneath shoreline sediments; indicating that groundwater was passing through marsh and subtidal deposits before reaching the estuary. Calculated discharge (6 to 10 liters per meter of shoreline per day) was small relative to fluxes of pore water across the marsh surface at those sites; even where discharge was maximal (at the upland border) it was 10 to 50 times less than infiltration into marsh soils. Pore water turnover in our marshes was therefore dominated by exchange with estuarine surface water. In contrast, new interstitial water entering subtidal sediments appeared to be primarily groundwater, discharged from below. The presence of fringing tidal marshes delayed transport and increased mixing of groundwater and solute as it traveled towards the estuaries. Soil-contact times of discharged groundwater were up to 100% longer in marshes than in subtidal shoreline sediments. Measured and modeled salinity profiles indicated that, prior to export to estuaries, the solutes of groundwater, marsh pore water and estuarine surface water were more thoroughly mixed in marsh soils compared to subtidal shoreline sediments. These findings suggest that transport of reactive solutes in groundwater may be strongly influenced by shoreline type. Longer soil-contact times in marshes provide greater opportunity for immobilization of excess nutrients by plants, microbes and by adsorption on sediment. Also, the greater dispersive mixing of groundwater and pore water in marshes should lead to increased availability of labile, dissolved organic carbon at depth which could in turn enhance microbial activity and increase the rate of denitrification in situations where groundwater nitrate is high.     

Forest harvest legacies control dissolved organic carbon export in small watersheds,western Oregon

This study examined physical and biological controls on dissolved organic carbon (DOC) fluxes from conifer-forest watersheds in the H.J. Andrews Experimental Forest of Oregon. We tested how DOC export was related to streamflow and legacies of wood on the forest floor three to five decades after harvest of old-growth forest in seven watersheds spanning the rain to snow elevation gradient. Three watersheds had old-growth forest and four had 30 to 50-year-old forest established after clearcutting of old-growth forest. Mean annual DOC flux in the watersheds was related to the biomass of forest floor wood, which was two or three times higher in watersheds with old-growth forest compared to young forest, and was inversely related to elevation, a measure of snowpack depth and duration. In contrast, fluxes of inorganic elements such as Si and Ca did not vary with harvest history or forest floor characteristics. Annual fluxes of DOC, Si, and Ca were linearly related to annual runoff, and annual volume-weighted concentrations of these ions declined by?<?0.6% with several-fold increases in annual runoff. Across all years, DOC concentrations peaked before the peak of the hydrograph in all watersheds, which we interpret as representing movement, likely via preferential and surficial flow, of organic materials mineralized and solubilized during the long dry summers in this ecosystem. DOC concentrations relative to stream flow exhibited clockwise hysteresis loops in each water year, also suggesting that soluble DOC produced in the dry summer is exported in the fall. DOC concentration differences between reference and harvested watersheds also peaked in late summer or early fall, suggesting that the source of the additional DOC from reference watersheds came from coarse woody debris that remains moist during the dry summers and that was significantly greater in watersheds with elevated DOC. Taken together, our results suggest that forest floor wood is a previously unappreciated control on the supply of DOC that can be exported, and runoff is a secondary control on total DOC flux to streams. The legacy of forest harvest on DOC flux can be observed for decades, as total ecosystem carbon stocks, especially coarse woody debris, may require centuries to develop after old-growth forest harvest.     

黄土丘陵沟壑区植被对不同空间尺度水沙关系的影响

根据晋西离石试验站的观测结果和相关文献的数据,研究了黄土丘陵沟壑区植被对坡面小区、全坡面、小流域及中大流域4个空间尺度水沙关系的影响。对梁峁坡坡面小区言,由于植被提高了土壤抗蚀性,因此植被不仅通过减水来减沙,也通过改变水沙关系来减沙。但在全坡面尺度,土壤侵蚀以沟蚀为主,由于植被措施难以改变沟道的各种水利参数,也难以有效控制切沟沟壁的重力侵蚀,导致水流进入沟道后仍然可以获取充足的泥沙,因此认为植被不会改变全坡面尺度的水沙关系。同样对于各级流域,由于植被措施难以改变沟道的输沙能力和黄土丘陵沟壑区流域泥沙来源充沛的特点,因此植被措施也不会改变其水沙关系,植被的减沙效应仅通过减水来实现。对离石试验站的一对水土保持对比沟的研究表明,即使在沟道已有茂密植被生长的情况下,高含沙水流的输沙能力也没有改变,这使得两者的水沙关系统计上可以认为完全一致。由于大流域水沙关系主要取决于沟道或河道的特性,而植被等坡面措施很难改变沟道或河道特性,因此认为流域尺度越大,植被越难以改变其水沙关系。     

聚丙烯酰胺对坡地苹果园水土流失和土壤养分流失的影响

为了改善坡地苹果园的土壤环境,遏制水土流失和土壤养分流失,探寻聚丙烯酰胺适宜的干撒施用量,2010—2012年在陕北丘陵沟壑区的坡地苹果园,以不施聚丙烯酰胺为对照,分别撒施0.6、0.8、1.0、1.2、1.4和1.6 g·m^-2的聚丙烯酰胺,监测果园地表产流、土壤流失、养分流失和苹果植株生长状况.结果表明: 坡地果园的径流量和5—7月的产流次数均随聚丙烯酰胺撒施量的增加呈“V”型变化,其中撒施量为1.0 g·m^-2时最低,但果园侵蚀泥沙量则随聚丙烯酰胺撒施量的增加而降低.地表径流和侵蚀泥沙中的铵态氮、速效磷和速效钾的浓度均随聚丙烯酰胺撒施量的增加而降低;聚丙烯酰胺可显著降低地表径流中的硝态氮含量,对侵蚀泥沙中的硝态氮含量则无显著影响;侵蚀泥沙中的有机质、全氮、全磷和全钾含量均随聚丙烯酰胺撒施量的增加而降低.聚丙烯酰胺提高了苹果单果质量及产量,但对苹果植株生长及果实风味品质无显著影响.聚丙烯酰胺在坡地苹果园中的适宜撒施量应为1.0 g·m^-2.     

The impact of urbanization on seasonal hydrologic and nutrient budgets of a small North American watershed

Present day hydrologic and nutrient budgets have been measured and presettlement budgets estimated for a small urban watershed in Madison, Wisconsin. The importance of different seasons and sources to the total loading were compared for past and present conditions. The seasonal budget revealed that while spring contributes the greatest loadings in both scenarios, summer exhibits the greatest change in loadings from past to present. As for sources this seepage lake receives most of its water and N from groundwater; however, most P comes from surface runoff. The greatest impact on the watershed since settlement appears to be the large increase in runoff matched by a decrease in evapotranspiration and subsurface flow. This shift in hydrology is largely responsible for the substantial increase in P loading and the decrease in the N : P ratio of total loading. The significance of the lake's modified morphometry to areal and volumetric loading is also explored.     

A Protocol for Conducting Rainfall Simulation to Study Soil Runoff

Rainfall is a driving force for the transport of environmental contaminants from agricultural soils to surficial water bodies via surface runoff. The objective of this study was to characterize the effects of antecedent soil moisture content on the fate and transport of surface applied commercial urea, a common form of nitrogen (N) fertilizer, following a rainfall event that occurs within 24 hr after fertilizer application. Although urea is assumed to be readily hydrolyzed to ammonium and therefore not often available for transport, recent studies suggest that urea can be transported from agricultural soils to coastal waters where it is implicated in harmful algal blooms. A rainfall simulator was used to apply a consistent rate of uniform rainfall across packed soil boxes that had been prewetted to different soil moisture contents. By controlling rainfall and soil physical characteristics, the effects of antecedent soil moisture on urea loss were isolated. Wetter soils exhibited shorter time from rainfall initiation to runoff initiation, greater total volume of runoff, higher urea concentrations in runoff, and greater mass loadings of urea in runoff. These results also demonstrate the importance of controlling for antecedent soil moisture content in studies designed to isolate other variables, such as soil physical or chemical characteristics, slope, soil cover, management, or rainfall characteristics. Because rainfall simulators are designed to deliver raindrops of similar size and velocity as natural rainfall, studies conducted under a standardized protocol can yield valuable data that, in turn, can be used to develop models for predicting the fate and transport of pollutants in runoff.