HYDROLOGY Hydrologic Storage
How much available water storage has been lost in the watershed since European settlement (1890-1900)?
Why is this important for hydrology?
Natural hydrologic storage consists of lakes and streams, wetlands, ponds, and other depressions that hold water for some period. Hydrologic storage has been altered by many human activities such as wetland removal, ditch construction, stream rechanneling and tile drainage.
These activities affect the way that the landscape stores and releases water. (Detenbeck et al., 1993; Hey and Phillipi, 1997; Staton et al., 2003; Kroger et al., 2008). These human activities to alter landscapes for other uses have come at the costs of increased peak flows, lower base flows, and increased nutrient and sediment concentrations in streams, rivers, and lakes (Mitch and Gosselink, 2007). Water quality usually is degraded when storage is removed, and improved when storage is added (Zedlar, 2003; Kovacic et al., 2006; Mitsch and Day, 2006).
CREATING THE INDEX
The index is based on two metrics. The first metric is a ratio of current water storage capacity to pre-settlement water storage capacity. Only the surface area of water features was considered.
Current wetland and waterbody storage / historic wetland and waterbody storage
Current wetlands and water bodies were used to estimate current storage. Current wetland area was obtained from the National Wetlands Inventory (NWI), 1974-1988 source photography, and Minnesota DNR 1:100K statewide data for maps lakes, rivers, streams, and other surface water bodies (DNR 100K). These data are cartographically aggregated to map all areas for each watershed.
Mapped historical water bodies, estimated historical wetlands, and a restorable wetlands inventory were used to estimate pre-settlement storage. Historic waterbody storage was estimated by modifying current maps of surface water bodies to include large-area lakes identified on Marshner’s and other pre- and early European settlement maps not present on current hydrographic maps, and removing constructed impoundments. Historic wetland storage was estimated based on hydric soils from SSURGO data, and a restorable wetlands inventory partially completed for Minnesota. These data were cartographically aggregated to estimate all hydrologic storage areas prior to 1900.
This ratio of current to historic (pre-1900) hydrologic storage was multiplied by 100 to create the first metric.
The second metric for this index is based on the meandered stream-to-straightened stream ratio. The overwhelming majority of ditches and stream straightening was constructed to reduce water storage and residence times on the land by removing stream meanders and floodplain access, or as new features created to convey water. Perennial stream and ditch lines were extracted from Minnesota DNR improved 1:24,000 based hydrography dataset. The length of stream segment between vertices was used to define a naturally meandering stream versus an artificially straightened stream. From the identified stream segments, a meandered stream / straightened stream ratio was calculated as:
stream miles / (meandered stream miles + straightened stream miles)
This ratio was then scaled from 0 to 100 using a threshold value of .5. Any ratio of .5 or lower was assigned a score of 0; a ratio of 1 received a score of 100. These rescaled scores created a second metric for the hydrologic storage index.
These two metrics were averaged to create a final score for hydrologic storage, which ranges from 0 (all historic storage capacity altered) to 100% (amount of storage unchanged).
The meandered/straightened stream ratio was also calculated to the catchment scale using the same line segment process describe above. The catchments were also scored on a linear scale using a ratio of .5 as a threshold and receiving a score of 0. The ratio of 1 receives a score of 100.
There is insufficient basis in scientific literature to numerically rank the combined index on a scale that relates the amount of conversion to specific quantitative thresholds of impairment. There are no known threshold values that correlate the percentage of remaining hydrologic storage cover and impacts to watershed hydrology. There is sufficient literature to conclude that storage reduction has substantial negative impacts, and that there is a qualitative relationship between the amount of hydrologic storage in a watershed and the quality, value, and health of aquatic resources. A threshold of 50% straightened streams was used as a 0 (degraded condition) for the meandered/straightened stream ratio.
An equal interval method was used to define levels of impact and display results.
Remaining hydrologic storage spanned almost the entire range of index values, from a low of 8 and several near 10, to high values of 100. Changes were generally greatest and scores lowest in watersheds in southern and northwestern Minnesota, and highest in the northeast.
The surface water storage index was generally lowest in the south-central and northwestern portions of Minnesota, and highest in the northeastern portion. This is not surprising, because the majority of wetland drainage and ditching is associated with agricultural land uses, and is concentrated in the south and northwest.
A total of 11 watersheds exhibited scores lower than 20, primarily in south central Minnesota and those watersheds largely near the main stem of the Red River of the North. Watersheds in the middle portion of the Minnesota River basin had low scores due to a high frequency of pre-settlement wetland vegetation, and substantial wetland removal and moderate to high concentrations of ditches. Low stream-to-ditch ratios and moderate to high rates of wetlands removal dominated watersheds in the Red River of the North.
Most remaining watersheds in the south-central and northwestern portions of Minnesota fell in the 20-40 and 40-60 categories, with various mixes of ditching and wetlands removal. The central portion of Minnesota, the extreme southeastern, and extreme southwestern watersheds exhibited scores from 60-80. These higher scores are due, in part, to fewer pre-settlement wetlands, steeper terrain, and less row-crop agriculture.
Scores from 80-100 dominated in the north central and northeastern portions of Minnesota, the largely forested portion of the state with little agriculture.
Land cover conversion to row-crop agriculture and urban development is correlated with decreased storage and more runoff through storm sewers, ditch drainage, and tiling. This reduced storage and increased runoff is associated with the delivery of sediment to streams and increased concentrations of nitrogen, phosphorus, endocrine disruptors and chemical contaminants from agricultural and urban systems.
The connectivity of land and water systems is affected by the loss of wetlands, riparian buffers and perennial cover. The pathways that moved energy, matter and organisms between these landscape features are disconnected, disrupted or shorted by constructed drainage features. This alteration can also increase storage in dams and reservoirs and create water flow between basins not historically connected.
Storage reduction and geomorphology may be related through changes in flow regimes. Decreased storage may lead to flashier streams, with higher peak flows that have higher erosivity and greater sediment transport capacity. Watercourses may become more incised with increased erosion driven by increased stream power.
Decreases in storage are directly related to losses in wetlands and obligate wetland species. This directly affects diversity, reproductive success, and population levels of fish, bird, and other taxa.
Scientific support for the components of the index is strong, with a number of studies showing increasing degradation to water quality and adverse change in stream hydrographs as storage is reduced in watersheds of the region.
A number of studies have identified the decrease in water quality and adverse changes in peak water flows with the loss of storage and the increase in ditching and tiling. Verry (1986) showed that runoff from tiled farmland in northern Minnesota may produce 100 times more sediment than from untilled, vegetated lands, while Detenbeck et al. (2000 and 2005) showed that drainage increased both peak flows and stream flashiness, with higher storm maximum flows and lower dry-period flows. Subsurface drainage, largely through tiling and ditching, affects the redistribution of water (Herzon and Helenius 2008), so that field sediment and soil-bound nutrient levels decrease, but water soluble nitrogen levels increase. Tiling may cause changes in geomorphology that affect downstream sediment loads. For example, Knox (1977) found a decrease in channel width caused by water table reductions following wetland drainage and channelization. Higher peak flows and increase channelization tends to increase stream bank erosion.
These changes may be somewhat mitigated by vegetation and farm management, but it appears to have a limited extent. Schilling et al. (2008) found that drainage affects flow even under re-established perennial vegetation, and produces higher water yields and peak flows relative to similar unchanneled or untilled areas. Alternative farming practices such as diverse crops, a larger proportion of perennial vegetation, and organic farming practices had some impact, but the tested alternative cropping practices with drainage still exhibited high peak flows in response to rainfall.
Research has also shown that adding storage, via wetlands and impoundments, and removing tiles, improves water quality. For example, many studies have documented a decrease in nitrogen and phosphorus exports after adding wetlands (Chescheir et al., 1991, Woltemade 2000, Kovak et al., 2000).
The confidence in the results is high. The basic relationship behind this index, between flashier streams and higher peak flows and increased reduction in storage, is well supported by a large number of scientific studies globally, nationally, in the Midwest, and in Minnesota. The evidence linking these two is incontrovertible, with theoretical and observational studies showing increase nutrient and sediment concentrations with reduced storage, and increases in flooding frequency and intensity. Studies by Detenbeck et al. (2005), show increased two-year peak flow and increasing flashiness with decreasing storage. Quist et al. (2007), showed longer recession times of water levels for farming practices with increased storage, and Schilling et al. (2008), showed an increase in stream flow regardless of vegetation with an increase in tile drainage. A decrease in storage increases channel depth and decreases channel width (Knox, 1977), and increases susceptibility to erosion (Birr and Mulla, 2001). Verry (1986), showed that surface runoff from tiled land may produce 100 times more sediment than from vegetation-covered land, and Herzon and Helenius (2008) noted that reduced storage and increased drainage through tiling and ditches leads to the loss of filtration through the soil, with increases in soluble nitrogen, phosphorus, and herbicides in surface waters.
Index scoring is on a linear scale, with increments of 20%. Work is sufficient to establish an increase in peak flows as remaining storage declines from 100% to 0% remaining pre-settlement storage. However, few studies have focused on response curve shape or thresholds, so there is little evidence to identify that these might exist.
Data used in this index are of adequate quality. National wetlands inventory data are available statewide, compiled on a high-resolution base. These data could be improved as there are frequent errors of omission, particularly for smaller wetlands, resulting in a tendency for underestimates for our calculated ratios. Stream data are well developed for Minnesota, however ditches and other drainage, particularly roadside ditches and tiling, are not well mapped, thus tending to inflate index scores.
The largest uncertainties are the mapped and estimated values in the denominator of the first component of this index. The pre-settlement Marschener map is relatively small-scale, and not as cartographically detailed as the more current hydrology data. It is unlikely this map leads to a serious under-estimation of pre-settlement lake extent, and there may be some overlap between represented lakes and subsequently built reservoirs. The estimates of pre-settlement wetlands are based on soils and terrain data, and in particular the terrain data could be improved to represent smaller basins in glacial landscapes. Both these datasets and the restorable wetlands data could be more rigorously evaluated, and should be in subsequent versions of this index.
Relationships between storage loss and subsequent impacts could be better quantified to identify when specific impairments are likely to be exceeded, and whether thresholds can be identified. Quantification of these relationships will likely be best approached for a small subset of important stream attributes, e.g., flow peak, nitrate or phosphorus concentrations, or some other widely measured and important characteristic.
The index could integrate increased costs or decreased values with changes in storage. For example, the increased per unit cost of increased flow in flood control and/or damage could be evaluated. This could be approached via modeling, e.g., model the flow with and without wetlands, then quantify impacts.
The index would benefit from improved mapping of wetlands, terrain, current streams, ditches, drainage tiles, wetlands, and constructed impoundments and the inclusion of water stored in soils, vegetation or stream channels. Depressions, flow direction, and basins are identified from the best available digital data. Most of these data were collected or depended directly on data collected from the 1950s through the 1990s, primarily wetland data estimated from aerial photographs and field visits in the 1970s through 1990s, elevation data and soils maps from the 1950s through early 2000s, and stream and ditch data from a combination of aerial photographs and maps compiled over this entire period. New wetlands data and new high-resolution image data may be coupled with high-resolution LiDAR data to improve the hydrography layers, and more accurately identify basins and other water storage features.
The index could be expanded to include an integrated water balance analysis. Runoff is precipitation minus evaporation/transpiration +/- storage. Given a network of stream gauges runoff timing and amount could be quantified, using the Indicators of Hydrologic Alteration (IHA); but this typically requires 5-10 years of observation. The length of the record for stream gages is too short for many watersheds, and the spatial distribution gages is currently too sparse to develop a statewide index, so a larger gage network, and longer time periods would be required.