The Rational Method is one of the most widely used methods for estimating peak discharge in small catchments. There are at least three forms of the Rational Method in use: deterministic, stochastic, and hybrid Rational Methods. These different forms are associated with distinct definitions of the runoff coefficient and produce distinct design flows, each of which has different risks associated with their exceedence. In this study, we firstly differentiate these forms of the Rational Method and show that a key point of difference between the forms lies in their interpretations of the runoff coefficient parameter. We then focus on the widely used hybrid Rational Method and demonstrate that the runoff coefficient is not only challenging to interpret, but is also dependent on land cover types, storm duration, and infiltration losses. With the understanding that most design manuals treat the runoff coefficient as a constant dependent on land cover and independent of storm properties, we explore the magnitude of error in design flows resulting from these assumptions. They suggest that the magnitudes of error associated with the conventional application of the Rational can be >500%. These issues with interpretation and internal consistency in the treatment of terms in the Rational Method suggest that it may not be possible to achieve reliable or consistent peak flow estimates using the Rational Method, motivating the use of more complex design tools.

中文翻译：

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不同方法学解释和径流系数使用对理性方法预测的影响

理性方法是估算小流域峰值流量的最广泛使用的方法之一。至少有三种形式的 Rational Method 在使用中：确定性、随机性和混合 Rational 方法。这些不同的形式与径流系数的不同定义相关，并产生不同的设计流量，每一种都有不同的风险与它们的超标有关。在这项研究中，我们首先区分了有理方法的这些形式，并表明形式之间的关键区别在于它们对径流系数参数的解释。然后，我们专注于广泛使用的混合理性方法，并证明径流系数不仅难以解释，而且还取决于土地覆盖类型、风暴持续时间和渗透损失。鉴于大多数设计手册将径流系数视为依赖于土地覆盖且独立于风暴特性的常数，我们探讨了由这些假设导致的设计流量误差的大小。他们认为与 Rational 的常规应用相关的误差幅度可能大于 500%。这些与 Rational 方法中术语处理的解释和内部一致性有关的问题表明，使用 Rational 方法可能无法实现可靠或一致的峰值流量估计，从而促使使用更复杂的设计工具。他们认为与 Rational 的常规应用相关的误差幅度可能大于 500%。这些与 Rational 方法中术语处理的解释和内部一致性有关的问题表明，使用 Rational 方法可能无法实现可靠或一致的峰值流量估计，从而促使使用更复杂的设计工具。他们认为与 Rational 的常规应用相关的误差幅度可能大于 500%。这些与 Rational 方法中术语处理的解释和内部一致性有关的问题表明，使用 Rational 方法可能无法实现可靠或一致的峰值流量估计，从而促使使用更复杂的设计工具。