Rainfall Losses: Difference between revisions

Latest revision as of 04:41, 21 July 2023

“The flood hydrologist is primarily concerned with:

"Interception by vegetation and subsequent evaporation or retardation when reaching the ground surface.
"Evaporation from ground surface during prolonged rainfall events or when accumulated in frozen form from snowfall, which is generally termed “sublimation.”
"Retention in surface depressions that act as miniature reservoirs that do not release their waters until their storage capacity is exceeded, and then only in relation to a stage versus discharge relationship comparable to an uncontrolled spillway on a reservoir.
"Infiltration into the receiving soil, rock, or combination thereof." ^[1]

"Any of the constituents of the Earth’s mantle have a capability to absorb water, whether it is a concrete airport parking and loading area or the most sandy of soils comprising some areas of the arid West. The first three of the above loss processes are usually low when compared with infiltration when rainfall intensities are sufficient to produce severe flood events such as the PMF. Under such conditions, the first three are often grouped with part of the infiltration loss and termed “initial losses,” and assumed to have been satisfied by antecedent rainfall occurring prior to the onset of the PMS."^[1]

"To illustrate the phenomena that occurs in the soil when water is applied in the form of rain, consider a condition at the onset of a rainstorm where the soil is comparatively dry as a result of no precipitation having recently occurred. Initially, part of the precipitation is intercepted by vegetation and, after the vegetation has reached its capacity to retain water by surface tension, additional precipitation simply runs off the leaves, stems, etc., and falls to the ground. Also, part of the rainfall falls directly on the ground surface and enters the soil. In nature, some of this precipitation evaporates back into the atmosphere; however, in the hydrologic analysis of floods, interception and evaporation losses are so small compared to the magnitude of the precipitation that they are neglected."^[1]

Best Practices Resources

National Engineering Handbook: Chapter 7 - Hydrologic Soil Groups, NRCS

National Engineering Handbook: Chapter 9 - Hydrologic Soil-Cover Complexes, NRCS

National Engineering Handbook: Chapter 10 - Estimation of Direct Runoff from Storm Rainfall, NRCS

Flood Hydrology Manual, USBR

Citations:

↑ ^1.0 ^1.1 ^1.2 Flood Hydrology Manual, Bureau of Reclamation, 1989

Revision ID: 7346
Revision Date: 07/21/2023

[Reclamation-1] 1.0 ^1.1 ^1.2 Flood Hydrology Manual, Bureau of Reclamation, 1989

[1]

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+[[Category:Rainfall Runoff Modeling]]
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+“The flood hydrologist is primarily concerned with:
+#"Interception by vegetation and subsequent evaporation or retardation when reaching the ground surface.
+#"Evaporation from ground surface during prolonged rainfall events or when accumulated in frozen form from snowfall, which is generally termed “sublimation.”
+#"Retention in surface depressions that act as miniature [[reservoirs]] that do not release their waters until their storage capacity is exceeded, and then only in relation to a stage versus discharge relationship comparable to an uncontrolled spillway on a reservoir.
+#"Infiltration into the receiving soil, rock, or combination thereof." <ref name="Reclamation">[[Flood Hydrology Manual| Flood Hydrology Manual, Bureau of Reclamation, 1989]]</ref>
-<!-- Introductory paragraph or topic page summary -->
-“Evaporation data is usually required for reservoir studies, particularly for low-flow analysis. Reservoir evaporation is typically estimated by measuring pan evaporation or computing potential evaporation”.<ref name="EM 1110-2-1420">[[Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420) | EM 1110-2-1420 Hydrologic Engineering Requirements for Reservoirs, USACE, 1997]]</ref>
-“Evapotranspiration (ET) is difficult to estimate because it is a complex process. It is determined by the atmospheric demand for water vapor (potential ET) and the availability of water to be evaporated. ET is a sum of pure evaporation from free water surfaces, such as wet vegetation, puddles, and lakes, and the transfer of soil moisture through plants and out their leaves (transpiration). The former process depends only on the atmospheric conditions (temperature, humidity, wind), whereas the latter also depends on plant characteristics (stomatal resistance) and on soil moisture availability”.<ref name="NEH210-630-20">[[National Engineering Handbook Hydrology: Chapter 20 Watershed Yield | National Engineering Handbook Hydrology: Chapter 20 Watershed Yield, NRCS, 2009]]</ref>
+"Any of the constituents of the Earth’s mantle have a capability to absorb water, whether it is a concrete airport parking and loading area or the most sandy of [[soils]] comprising some areas of the arid West. The first three of the above loss processes are usually low when compared with infiltration when rainfall intensities are sufficient to produce severe flood events such as the PMF. Under such conditions, the first three are often grouped with part of the infiltration loss and termed “initial losses,” and assumed to have been satisfied by antecedent rainfall occurring prior to the onset of the PMS."<ref name="Reclamation"/>
-“Many models are available for estimating potential evapotranspiration from meteorological data (Jensen, Burman, and Allen 1990; ASCE 1996). They vary in their assumptions, the processes described, the input data required, and the temporal scale for which they are appropriate. Potential ET can also be estimated from pan evaporation data if suitable pan coefficients are available” <ref name="NEH210-630-20" />
+"To illustrate the phenomena that occurs in the soil when water is applied in the form of rain, consider a condition at the onset of a rainstorm where the soil is comparatively dry as a result of no [[precipitation]] having recently occurred. Initially, part of the precipitation is intercepted by vegetation and, after the vegetation has reached its capacity to retain water by surface tension, additional precipitation simply runs off the leaves, stems, etc., and falls to the ground. Also, part of the rainfall falls directly on the ground surface and enters the soil. In nature, some of this precipitation evaporates back into the atmosphere; however, in the hydrologic analysis of floods, interception and evaporation losses are so small compared to the magnitude of the precipitation that they are neglected."<ref name="Reclamation"/>
-“Even if potential ET is adequately estimated, the actual ET is less than or equal to this amount and depends primarily on soil moisture availability. Because of this interplay between the atmospheric demand and the soil moisture, determining the actual ET is problematic without a detailed hydrologic model operated at a short time step (i.e., a day or less). If adequate assumptions can be made, however, reasonable estimates of actual ET as a fraction of potential ET are possible”.<ref name="NEH210-630-20" />
 ==Best Practices Resources==
-{{Document Icon}} [[Flood Hydrology Manual | Flood Hydrology Manual (Bureau of Reclamation)]]
+{{Document Icon}} [[National Engineering Handbook: Chapter 7 - Hydrologic Soil Groups | National Engineering Handbook: Chapter 7 - Hydrologic Soil Groups, NRCS]]
-{{Document Icon}} [[National Engineering Handbook: Chapter 7 - Hydrologic Soil Groups | National Engineering Handbook: Chapter 7 - Hydrologic Soil Groups (Natural Resources Conservation Service)]]
+{{Document Icon}} [[National Engineering Handbook: Chapter 9 - Hydrologic Soil-Cover Complexes | National Engineering Handbook: Chapter 9 - Hydrologic Soil-Cover Complexes, NRCS]]
-{{Document Icon}} [[National Engineering Handbook: Chapter 9 - Hydrologic Soil-Cover Complexes | National Engineering Handbook: Chapter 9 - Hydrologic Soil-Cover Complexes (Natural Resources Conservation Service)]]
+{{Document Icon}} [[National Engineering Handbook: Chapter 10 - Estimation of Direct Runoff from Storm Rainfall | National Engineering Handbook: Chapter 10 - Estimation of Direct Runoff from Storm Rainfall, NRCS]]
-{{Document Icon}} [[National Engineering Handbook: Chapter 10 - Estimation of Direct Runoff from Storm Rainfall | National Engineering Handbook: Chapter 10 - Estimation of Direct Runoff from Storm Rainfall (Natural Resources Conservation Service)]]
+{{Document Icon}} [[Flood Hydrology Manual | Flood Hydrology Manual, USBR]]
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