1.4 Water Quality Models
（土建学院2012级水利班 高毛毛、刘畅畅）
Because water quality is inextricably linked to water quantity, it is important for the hydrologist to understand the significance of developing modeling techniques that can accommodate both features.
因为水质与水量存在必然联系，所以需要开发建模技术以满足这两种水文特征的需要，清楚这项工作的意义对于水文学家来说是十分必要的。
A water quality model is a mathematical statement or set of statements that equate water quality at a point of interest to causative factors. In general, water quality models are designed to (1) accept as input, constituent concentration versus time at points of entry to the system,(2) simulate the mixing and reaction kinetics of the system, and (3) synthesize a time-distributed output at the system outlet.
水质模型是一个或一组描述某点水质与其影响因子的数学公式。一般而言，水质模型的设计包括：1、输入系统入口处的时间一组分浓度变化过程2、模拟系统中的混合反应机制3、综合生成系统出口处时间一组分过程。
Either stochastic (containing probabilistic elements) or deterministic approaches may be taken in developing methods for predicting pollutional loads. The former technique is based on determining the likelihood (frequency) of a
particular output quality response by statistical means. This is similar to frequency analysis of floods or low flows. Water quality records should be available for at least 5 years (and preferably much longer) if estimates of return periods for infrequent events are to be reliable.
可以用随机性的方法（包括概率因子）或者确定性的方法来开发污染物荷载预测的方法。前者利用统计方法确定一种特定水质输出的可能性（频率）。这类似于洪水与枯水频率分析。水质记录需要至少五年才能保证对于小概率事件重现期评估的可靠性。（水质记录时间越长越好）。
The deterministic approach (output explicitly determined for a given input) requires that a model be developed to relate water quality loading to a known or assumed hydrologic input. Such a mode can range from an empirical concentration discharge relation to a physical equation representing the hydrochemical cycle. The ultimate modeling technique is that which best defines the actual mechanism triggering the water quality response. The cause of a given state of pollution can then be specifically identified.
确定性方法（给定输入得到确定性输出）需要一个模型把水质负荷和已知的或假定的水文输入联系起来。这样的模型可以从一个浓度流量的经验关系也可以是一个表征水化学循环的物理方程。最好的建模方法是能够描述引起水质反应机制的方法，这样在污染一定情况下，其原因就可以具体确定。
Water quality models vary in their complexity. Their nature depends on the application to be made of the model, the availability of data, and the level of
understanding of the hydrochemical and hydrobiological processes involved. Unfortunately, the complexities of these processes, which are great, make the difficulties associated with hydrological modeling seem small in comparison.
水质模型的复杂程度各不相同。模型的性质取决于他们的用途，资料的可得性以及对所涉及的水文化学、水文生物学的理解程度。与水文模型相比，水质模型的建立过程更加复杂。
In general, water quality models should permit acceptance of inputs in terms of pollutant (constituent) concentration versus time at points of entry into the system, description of the mixing and reaction kinetics in the stream element or groundwater element of concern, and synthesis of a time-distributed output indicating pollutant concentration at the outlet of the element segment being modeled. An analogy may be drawn to the stream flow routing, which is performed in a downstream sequence from one stream channel segment to another. In the case of water quality modeling, the common representation is the calculation of change in constituent concentration as it passes through successive states of the water body being modeled.
一般来说，水质模型应考虑输入到系统各控制点的污染物（组分）浓度随时间变化过程，描述所关注的河段或地下水体中污染物的混合和反应机制，综合模拟系统输出断面污染物随时间的分布结果。与水文汇流（即计算流量随水体连续流动的变化）类似，水质模型是计算污染物浓度随水体连续流动的变化。
As in the case of other water resources modeling processes, the approach may
be deterministic or stochastic. In the case of water quality models, the stochastic approach is often ruled out because actual records of water quality parameters are unavailable for long enough periods to permit frequency methods to be used. Of course, generated sequences can be used for this purpose if adequate mathematical statements representing the kinetics of the system can be developed and their parameters determined.
正如其他水资源建模一样，建模方法可以是确定性的或者随机性的。对于水质模型而言，通常不采用随机性方法，因为采用频率方法需要足够长的水质实测资料，而这些资料是很难获得的。当然，如果有足够的数学公式来描述系统运动机制，并且其参数可以确定的话，也可以采用生成序列来进行模拟。
The deterministic approach to water quality modeling requires that relations between water quality loading and the flow or hydraulic features of the system be established and that the appropriate chemical and/or biological reactions be tractable for solutions. Where theory based relations cannot be employed, empirical relations are often used. The optimum model to use would be the one best defining the actual water quality response of the system. Many models have been developed.
确定性水质模型需要建立水质负荷与水流或其水力学特征之间的关系，并且要求水中相关的化学和生物反应是可以追踪的。当这种关系不能由理论推导得来的话，可采用经验关系。能够最好地定义系统实际的水质反应的模型才是最优的水质模型，现在已经开发了多种水质模型。
Pollutants may be classified as conservative or nonconservative (constituents having time-dependent decays); somewhat more specifically as organic, inorganic, radiological, thermal, or biological; and finally they may be categorized by specific forms such as BOD, phosphorus, nitrogen, bacteria, viruses, and specific toxic substances. These pollutants may be loaded into a water course or groundwater system from either point or nonpoint sources.
污染物可以分为守恒的和非守恒的（组分随时间衰减）两种；更具体点儿，还可分为有机的、无机的、放射性的，与热有关的或者生物的；另外还可按照某些特定的形式，把其分为生化需氧量、磷、氮、细菌、病毒和特殊有毒的物质。这些污染物可以以点源污染或非点源污染的方式进入到水道或者地下水中。
The time rate of delivery of a pollutant must be determined if its characteristics are to be modified by management practices or its impact on some element of the system evaluated. For example, the consequences of some quantity of silt delivered to a lake would not be the same if it were introduced over a period of 5 days as opposed to 2 hr. Thus monitoring of water quality must generally be on a continuous basis if the data are to be of value for water resources planning and/or developing continuous modeling processes.
如果我们需要用管理措施来调整污染物的扩散特征或者评估其对系统某些元素造成的影响，必须确定污染物的运送速度。例如，一定量的淤泥在5天之内和2小时之内进入湖中，其产生的水体的含沙量是不同的。如果水质监测数据需要应用于水资源规划或开发连续性模型，水质监测需要连续进行。
Unstable pollutants such as radioactive materials, heat, biochemical oxygen demand, and living organisms all have time-dependent decays and are thus nonconservative in nature. For dealing with such constituents, it is necessary that both the mixing properties and the reaction kinetics of the system be approximated. On the other hand, many inorganic pollutants are conservative in nature and their handling depends mainly on an ability to model the mixing mechanics of the receiving body of water.
不稳定的污染物，如放射性物质、热量、生化需氧量及其生物，都随时间衰减，所以，本质上都是非守恒的。在处理这些成分时，应该给出其近似的混合特性以及反应机制。另一方面，许多无机污染物在本质上是守恒的，它们的处理主要依靠接受水体混合机制的建模能力。
The problems associated with modeling chemical and biological changes in a water body are many and complex. The field conditions encountered in natural systems are highly varied and often negate the validity of reaction rate and other mechanisms determined under laboratory conditions. Furthermore, pollutants derived from nonpoint sources are subjected to many alterations in their travels over and/or through the ground before they reach a water course. The highly varied chemical, biological, and hydraulic characteristics of the land must be dealt with in estimating pollutant loadings from these sources. Although qualitative descriptions of hydrochemical and hydrobiological processes are easy to come by, their quantification is something else again. Fortunately, in some case, empirical relations between pollutant concentration and stream flow or other hydrologic variables can be used to describe water quality loading mechanisms that cannot
be obtained on a more theoretical basis. It should be stressed, however, that a model is only as good as the data and theory on which it is based.
有关在水体中模拟化学和生物变化的问题许多，而且复杂。在自然系统中遇到的现场条件是非常多变的，并且经常会导致实验室条件下确定的反应速率以及其他机制失效。此外，来自非点源的污染物在到达水道之前的过程中会发生许多变化。多变的化学、生物，与地形有关的水力特征必须在从这些来源估计的污染物含量中涉及。虽然性质上关于水文化学和水文生物过程的描述是容易得到的，但是定量上不是这样。幸运的是，在某些情况下，有关污染浓度和河流水量或其他水文变化的经验关系可以被用来描述不能在一个更加理论的基础上得到的水质负荷机制。然而，模型只和它所依赖的理论和数据一样好，这一点应该被强调。
