Chow (1964) called the year 1930-1950 the Period of Rationalization since it is the era where a lot of inventors produced a significant step forward for the field of hydrology; as government agencies began to develop their own programs of hydrologic research. The period of rationalization (1930—1950) is the period during which we find the true gurus of modern hydrology. Advances like L.K. Sherman's unit hydrograph (1932), R.E. Horton's work on infiltration (1933), groundwater studies by C.V. Theis (1935), and flood frequency statistics by E.J. Gumble (1941) and A. Hazen (1930) are examples of contributions during this period. These pioneers published their research and developed procedures that are still very much in use today (Chow 1964).
The following are the important research outputs during the 1930-1950:
· Sherman’s unit hydrograph (1932)
The Unit Hydrograph (UH) of a watershed is defined as the direct runoff hydrograph resulting from a unit volume of excess rainfall of constant intensity and uniformly distributed over the drainage area. The duration of the unit volume of excess or effective rainfall, sometimes referred to as the effective duration, defines and labels the particular unit hydrograph. The unit volume is usually considered to be associated with 1 cm (1 inch) of effective rainfall distributed uniformly over the basin area.
· Horton’s infiltration theory (1933)
This states that for excess rates of rainfall, the actual infiltration rate will follow the limiting curve. The capacity decreases with time and ultimately reaches a constant rate, caused by filling of soil pores with water, which reduces capillary suction.
· Theis’s non-equilibrium equation (1935) in well hydraulics
Theis’s non-equilibrium equation (1935) introduced a groundbreaking tool for determining the hydraulic properties (transmissivity and storativity) of nonleaky confined aquifers. Analysis with the Theis method is performed by matching the Theis type curve to drawdown data plotted as a function of time on double logarithmic axes.
· Gumbel (1958) proposed the use of extreme-value distributions for frequency analysis of hydrologic data, thus forming the basis for modern statistical hydrology.
During this period, the U.S. Army Corps of Engineers (ACOE), the NWS within NOAA, the U.S. Department of Agriculture (USDA), and the USGS made important contributions to the theory of hydrology and the establishment of a national network of precipitation, evaporation and calculation of stream flow. The NWS is still largely responsible for rainfall measurements, reporting and forecasting of severe storms and other related hydrologic investigations.
During this time, the U.S. ACOE and the USDA Soil Conservation Service (now known as the Natural Resources Conservation Service [NRCS]) made important contributions to the field of hydrology including flood management, river growth, irrigation and soil conservation. More recently, for both quantity and quality data, the USGS has taken major measures to develop a regional network of stream gages and rainfall gages. In providing the study of dynamic hydrologic data to establish relationships and clarify hydrologic processes, their water supply publications and special studies have done much to advance the field of hydrology. The NWS and USGS both support numerous websites for the dissemination of watershed information and precipitation and streamflow data from thousands
The following are the advancements in the field of hydrology for the following period.
1950s - 1960s
· Increase of urbanization following World War II in the United States and Europe led to better methods for predicting peak flows from floods, for understanding impacts from urban expansion, and for addressing variations in storage in water supply reservoirs.
· Major expansion of cities and water systems within the United States during the 1950s led to a need for better understanding of floods and droughts, especially in urban areas.
· Water resource studies became an everyday occurrence in many rapidly developing areas of the United States, tied to the expansion of population centers in the southern, southwestern, and western states.
1970s - 1980s
· The evaluation and delineation of floodplain boundaries became a major function of hydrologists, as required by the Federal Emergency Management Agency (FEMA) and local flood control or drainage districts. In order for communities to be eligible for flood insurance administered by FEMA, they are required to delineate floodplain boundaries using hydrologic analysis and models. This function has taken on a vital role in many urban areas, as damages from severe floods and hurricanes continue to plague the United States, especially in coastal and low-lying areas.
1980s - 1990s
· Late 1990 first detailed global water resources assessments comparing water availability with water use (Shiklomanov, 1997). Mostly relied on statistics of water use (e.g., AQUASTAT) and observations of hydrology. Shortly thereafter, first Macro scale hydrological models (MHMs), WaterGap (1997), WBM (1998) and MacPDM (Arnell, 1999).
· Minimum stream power theory (Chang 1980)
· Maximum sediment discharge and Froude number theory (Ramette 1980)
· Maximum sediment discharge theory (White et al. 1982)
· Maximum friction theory (Davies and Sutherland 1983)
· Minimum unit stream power theory (Yang and Song 1986)
· Thermodynamic theory (Yalin and da Silva 1997, 1999), minimum energy dissipation theory (Rodriguez-Iturbe et al. 1992)
1990s – 2020
· Principle of least action (Huang and Nanson 2000)
· Entropy theory (Deng and Zhang 1964; Singh et al. 2003a, b; Singh and Zhang 2008a, b). Each theory leads to unique hydraulic geometry relations, meaning different values of exponents.
· Singh (2003) has discussed characteristics of these relations with regard to their basis, tendency to equilibrium state, limitations of the equilibrium assumption, validity of power relations, stability of exponents in power relations, effect of channel patterns, effect of stream size, dependence of exponents on climatic and environmental factors and land use, extension to drainage basins, and impact of boundary conditions.
These are of the some computer advances in hydrology since 1960s to present.
· 1960-1970- The introduction of the digital computer into hydrology during the 1960s and 1970s allowed complex water problems to be simulated as complete systems for the first time.
· 1970- Hydrologic computer models developed in the 1970s have been applied to areas previously unstudied or only empirically defined. For example, urban stormwater, floodplain and watershed hydrology, drainage design, reservoir design and operation, flood frequency analysis, and large-river basin management have all benefited from the application of computer models.
· Single-event models such as HEC-HMS are used to simulate or calculate the resulting storm hydrograph (discharge vs. time) from a well-defined watershed area for a given pattern of rainfall intensity.
· Continuous models such as the Hydrological Simulation Program Fortran (HSPF) and the Storm Water Management Model (SWMM) can account for soil moisture storage, evapotranspiration, and antecedent
· Rainfall over long time periods. Statistical models can be used to generate a time series of rainfall or streamflow data, which can then be analyzed with flood frequency methods.
· Newer distributed hydrologic models (i.e., VFLO and the MIKE series of models) can handle input, output, and data manipulation at the watershed level.
· 1970- Unquestionably new digital approaches combined with distributed terrain modeling have revolutionized hydrology in recent years, just as the original wave of models did in the decade of the 1970s.
· The data revolution in hydrology and geographical information systems (GIS) have made available newer and more accurate datasets on topography, slope, rainfall, soils, land use, and channel characteristics for many areas. Moreover, most hydrological and meteorological data may be retrieved online from agencies such as the USGS and NWS, and various county and municipal sources.
G Johnston (2005). HYDROGRAPH Sherman (1932) first proposed the unit hydrograph.
Retrieved from https://studylib.net/doc/7268286/hydrograph-sherman--1932--first-proposed-the-unit-hydrograph
V Singh (2018). Hydrologic modeling: progress and future directions. Retrieved from
P Bedient et. al (2013). Hydrology and Floodplain Analysis.