Unit 2 Hydrology_Lecture 1 - Hydroperiod and Hydropattern

Unit 2 Hydrology_Lecture 1 - Hydroperiod and Hydropattern -...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Wetlands and Water Quality Unit 2: Hydrology Lecture 1: Hydroperiod and Hydropattern Central OH. (Paul Weihe) Hydrology-Principal Environmental HydrologyForce Dictates Oxygen availability Biogeochemical cycles Organism survival and selection Temperature regulation Transport of matter In part determines what kind of wetland develops. Hydrology is made up of several components each of which is influential on community structure and biogeochemical processes. 1 Hydroperiod and Hydropattern Hydroperiod is a term that describes the number of days in a year that a particular wetland or location in a wetland is flooded. For instance a hydroperiod of 180 days would indicate that water was standing at a given location in the wetland for 180 out of 365 365 days of the year. Or a plant species that could tolerate a 60 day hydroperiod would be one that could withstand flooding for 60 day's out of the the year. "Hyroperiod" lumps together two important components of hydrology Frequency and Duration that should be considered separately, and does not take into consideration three critical components. The more descriptive term to describe hydrology is Hydropattern. Hydropattern Duration Depth Wetland Community Structure and Biogeochemical Function Frequency Flow Timing or Season Frequency The term frequency refers to how many times flooded or saturated conditions occur in a wetland over a given time period. In the case of a tidal wetland this might be once every 12 hours, or 365 time per year. In ephemeral wetlands such as vernal pools in California with a pronounced wet and dry season it might be once per year. In the extreme you may have heard about the 25, 50 or 100 year floodplain. This describes the statistically determined probability that a storm event of a given magnitude will occur. 2 Duration The duration of flooding or saturation relates to the length of time a single flooding event or saturated condition last. For instance, if water levels in a wetland came up after a storm event and flooded a plant on March 3, then drained and exposed the plant on March 20, the duration of flooding would be 17 days. days. If a second flood event occurred in April and flooded the plant between April 5 and April 10, the duration of that event would be be 5 days. This is not the same as Hydroperiod. Each of these events is considered separately. Hydroperiod on the other hand combines duration of all events and only reports one number for the year. Based on Hydroperiod, the above example would have a 22 day hydroperiod for that particular year, yet the duration would be and average of 11 days with max of 17days and min of 5 days. Depth The term depth relates to the depth of the water within a wetland or a given point within a wetland during a flood event. In non human regulated systems, depth is very dynamic and never stays at the same level for long. Therefore, flooding depth is often based on averages, maximums and minimums. In treatment wetlands or wetlands adjacent to water bodies with control structures, flooding depth is often far more consistent and can have consequences on vegetative recruitment and development. Timing or Season Timing or Season of flooding relates to the period of the year when when inundation or saturated conditions are likely to occur. This becomes critical when we look at the mechanism of influence hydrology has on development of vegetation or microbial processes. Respiration rate of an organism, if greater than oxygen supply, will result in hypoxic conditions and potential mortality of the organism. Temperature is a major component regulating metabolism of poikilothermic poikilothermic organisms, Therefore, the time of year when flooding occurs relative to the time of year when organism would be most active becomes important. This relates back to the reference to "growing season" in some of the of wetland definitions where the timing of flooding must occur during an active during phase of vegetative growth for the effects of hydrology to be expressed. expressed. 3 Flow Flow relates to the movement of water. Although intuitive in riverine floodplains, the concept of flow is becoming increasingly important in our understanding of the development of other systems. Effects of flow are numerous Sediment transport, scour Mixing the water column resulting in the transfer of oxygen from the surface rich layer to the oxygen deprived soil/water interface interface through convective vs. diffusive processes. Disrupts thermalclines that may develop between surface and subsurface waters Transports nutrients, reducing boundary layers that might otherwise result in nutrient limiting conditions. "Visualizing" Hydropattern Quantifying the five components of hydropattern in a wetland is a daunting task, yet graphing the stage (water level) in a wetland over time allows us to relatively easily quantify four of the five components of Hydropattern. Hydrograph A Hydrograph is simply the measure of water depth or volume over time. This measure helps us characterize the hydrologic condition of a wetland, or other water body The hydrograph in the next slide represents a single event. However, hydrographs can also represent the integration of multiple events and the cumulative effect of sources and losses effecting a wetlands water budget. 4 Rainfall Rising limb Storm Runoff Hydrograph Recession limb Base flow "Signature" hydrographs Each storm event has a signature hydrograph, each watershed has a signature response to a rain event. If water depth in a wetland is measured over time the integration of flooding and draining events produces a hydrograph that is very informative and often predictive of flora, fauna, and biogeochemical processes one might expect to occur in the wetlands. Therefore, we might also say that wetlands have a signature hydrograph. Wetlands of similar vegetative community type tend to have similar hydrograph characteristics. 5 Variation in Tidal Flood Frequency Daily Tides Mixed Tides Semidaily Tides 6 Frequency and amplitude of tides in North and Central America Ross; 1972 Undeveloped Watershed Pervious surface Depressions in landscape Vegetated "rough" surface Dry Rain Event Effect of Development on Storm Hydrograph Stage Storm Event Pre-development Time 7 Developed Watershed Impervious surfaces increase Depressions in landscape drained Vegetated "rough" surfaces decrease and become smooth with quicker runoff. Culverts, swales, and ditches move water off the landscape quickly. Rain Event Effect of Development on Storm Hydrograph Stage Post-development Storm Event Time Effect of Development on Storm Hydrograph Post-development Stage Storm Event Pre-development Time 8 Interpreting a Hydrograph By dissecting a hydrograph for frequency, depth, duration and timing we can quantify most of the components of hydropattern. Depth from hydrograph 3 2 1 Depth, ft 0 -1 -2 OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP Max depth 2.9 ft Average Depth 0.8 ft. Soil Elevation 9 Frequency from hydrograph 3 2 1 Depth, ft 0 -1 -2 OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP Soil Elevation 4 flooding events Frequency = 4 times / year 1 2 3 4 Duration from hydrograph 3 2 1 Depth, ft 0 -1 18+ days 43 days Maximum Duration = 137 days Average Duration = 55 days Hydroperiod = 218 Soil Elevation -2 OCT NOV DEC JAN FEB 137 days 20 days MAR APR MAY JUN JUL AUG SEP Timing / Seasonality of Flooding from Hydrograph 3 2 1 Depth, ft 0 -1 -2 OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP Soil Elevation flooding = late winter and spring 10 Terms used to describe hydropattern of Non-tidal Wetlands NonPermanently Flooded: Flooded: Intermittently Exposed: Exposed: Flooded through the year in all years Flooded throughout the year except in years of extreme drought Flooded in the growing season in most years Semi-permanently Flooded: SemiFlooded: Seasonally Flooded: Flooded: Flooded for extended periods during the growing season, but usually no surface water by end of growing season substrate is saturated for extended periods in the growing season, but standing water is rarely present flooded for brief periods in the growing season, but water tables otherwise well below surface surface is usually exposed with surface water present for variable periods without detectable seasonal pattern Saturated: Temporarily Flooded: Intermittently Flooded: Flooded: Flood duration Terms used to describe hydropattern of Tidal Wetlands Subtidal: Subtidal: Irregularly Exposed: Exposed: Permanently flooded with tidal water Surface exposed by tides less often than daily Regularly Flooded: Flooded: Alternately flooded and exposed at least once daily Irregularly Flooded: Flooded: Flooded less often than daily Flood duration Hydroperiod vs. Hydropatten Hydrograph Depth, Duration, Frequency, Flow, Timing 11 ...
View Full Document

This note was uploaded on 03/01/2010 for the course PCB 4683 taught by Professor Williams,j during the Spring '08 term at University of Central Florida.

Ask a homework question - tutors are online