Despite the popular image of "sunny southern California," the study region is a land of severe floods, the intensity and frequency of which will probably increase in response to upstream development. When land is converted from natural vegetation to built structures, the area of impervious ground cover and the velocity at which water runs off the land usually increase. Infiltration of precipitation into the soil decreases, causing more runoff and flooding during storms and less long-term recharge of soil moisture and aquifers. This process generates changes in hydrological regime in the associated terrestrial and downstream riparian and aquatic systems. Furthermore, floods of greater magnitude erode more sediment in areas of highly erodable soils --which are prevalent in this region-- and also deposit more sediment downstream, thus changing riparian conditions.
Hydrological regime (i.e., the quantity, timing, location, and quality of available surface water, soil water, and groundwater) is one of the major factors controlling biodiversity in southern California. Hydrological regime is defined in terms of both single events and long-term patterns. The single-event perspective describes the flood that results from a rainstorm of some intensity, duration, and frequency that falls in a drainage basin with moisture already in the soils. The water that does not infiltrate the soil or evaporate into the atmosphere flows to and through the stream channel. If the volume of water exceeds the capacity of the channel to carry it, a flood results. Upstream land-use changes that lead to increased impervious areas usually increase peak discharge (discharge is the volume of water that flows past a point in the stream) and always increase total storm discharge.
The long-term perspective encompasses the precipitation from many storms, accounts for water losses from a basin via evapotranspiration and deep seepage, and examines daily or monthly fluctuations of soil moisture and streamflow, possibly for many years. This perspective allows estimates of water budgets for upland soils, as well as hydroperiods (length of time of standing water or saturated soil conditions) in riparian and floodplain areas. Procedures for simulating both long-term hydrological and single-event regimes are shown in the diagram in figure 25.
The relationship between hydrologic regime and vegetation, and thus biodiversity, can be interpreted and predicted as a consequence of either soil water availability or hydroperiod. Water availability controls edaphic factors for plant communities, which in turn provide habitat for animals. Thus, alterations to the hydrological regime will necessarily affect biodiversity, often in places distant from the source of the change.
Figure 26 shows the seven drainage basins, or watersheds, that flow through or are adjacent to Camp Pendleton and empty into the Pacific Ocean. For purposes of analysis, these basins have been further subdivided into 113 third-order stream basins which are approximately the same size of towns and larger developments. The basins in this study are, from north to south, the San Juan River, several small creeks combined into the North Coastal Drainages, the San Mateo River, San Onofre Creek, the south coastal drainages including Las Pulgas Creek, the Santa Margarita River, and the San Luis Rey River. Headwaters for the main watersheds of the Camp Pendleton region originate on the western slopes of the Peninsular Range. The Santa Margarita River, the largest stream basin in the study area, flows southwest to the Pacific Ocean. Below the confluence of Murrieta and Temecula Creeks, the Santa Margarita is also southern California's only "free-flowing" river; it has no major dams.
In some cases, such as the south coastal drainages and San Onofre Creek, the basins are completely enclosed by Camp Pendleton. In other cases, especially the Santa Margarita and San Mateo Rivers, most of the basin is upstream of the base and is under intense pressure for development.
Four important estuaries are on the base at the mouths of the Santa Margarita, San Onofre, Las Flores, and San Mateo Rivers, and minor estuaries exist at the mouths of all the coastal drainages. Two bordering river basins interact with Camp Pendleton in less significant ways, either by receiving treated sewage effluent (San Luis Rey River) or by encroaching slightly into Camp Pendleton property (San Juan River).
The lower portions of the Santa Margarita and San Onofre watersheds are characterized by alluvial valleys containing the principal source of water for Camp Pendleton. Camp Pendleton's domestic, agricultural, and industrial water supply is totally dependent on pumping from underground aquifers located on the base, which are recharged by percolation from rivers and streams. At present, Camp Pendleton has no direct connection to imported water, unlike most other places in southern California.
Long-term soil moisture budget and streamflow modeling define the relationships between vegetation types and soil-water budget and riparian hydroperiod, respectively. A soil-moisture index, defined as the total volume of water that flows through a 30m by 30m GIS grid cell during a year, divided by the tangent of the slope, is calculated (modified from O'Loughlin, 1986, Barling, et al., 1994).
Figure 27 shows the soil moisture index in the 1990+ base year; figure 28 shows the soil moisture index resulting from Plans Build-Out; and the bar chart in figure 29 shows their differences. Although more water runs off the upland areas that are developed, the entire study region, in general, becomes slightly wetter after Plans Build-Out, because water runs through the lowlands in greater volume. Thus, the upland soils become slightly drier but the lowland and riparian soils become much wetter.
The consequences of this pattern of soil-moisture change on vegetation are subtle, but they indicate potential risk to the maintenance of current vegetation patterns, and thus biodiversity. In general, drier upland vegetation that is not converted to urban uses in Plans Build-Out will slowly grow into oak woodlands (see Callaway and Davis 1993 for evidence that this actually happens when fires are suppressed). Increases in downstream and lowland soil moisture and flooding could mean increases in the extent of exotic riparian vegetation, especially invasion by Arundo donax, as greater floods decimate existing riparian areas, leaving bare sediment available for colonization by opportunistic plant species.
The potential for large floods is particularly high in this southern California region because of the extreme variability of precipitation and runoff. For example, successive soil-saturating storms in early 1993, combined with intense rainfall (175mm in 24 hours) in the upper watershed, led to record flooding in the Santa Margarita River Basin on January 16, 1993. At the damaged gaging station at Ysidora, the estimated peak discharge may have been greater than 1870m3/sec, the highest for the 68 years of record. The 100-year peak flow at this gauging station is estimated (for flood control purposes) to be 2830m3/sec and assumes that upstream storage sites are full, with a total five-day flood volume predicted to be 178,000,000m3. This flood-control assumption does not consider the effects of expanded urbanization in the upper watershed, which is likely to cause large floods to become more frequent, and the largest floods to become even larger.
During the summer months in this Mediterranean climate, the frequency of extremely low flows in unregulated streams is particularly high. It is common for the San Mateo, San Onofre, and Los Flores Creeks to be dry from July through October. The Santa Margarita River has had no flow reaching the Pacific during about 26% of the period of record. Most of the tributaries to these rivers are also intermittent.
Floods produced by single-storm events are simulated by two methods, depending on the size of the basin. The first method, which is used for the third-order basins and is not spatially explicit, was originally developed by the U.S. Soil Conservation Service, now named the NRCS. This method uses the NRCS designation of hydrological soil group (HSG) for each named soil series to assign a Runoff Curve Number (RCN) to areas with characteristic combinations of soil and land cover. RCN is a variable used to calculate infiltration and runoff volumes from a given rainfall amount. Figure 30 is a map of RCN generated for the 1990+ land cover. Figure 31 shows the RCN in Plans Build-out. The models then calculate the total volume of runoff produced by a storm of a given intensity, and the runoff volume is then routed overland and through channels according to overland and channel flow models. Storm hydrographs (plots of discharge vs. time) are then calculated by summing the volume of water reaching a point of interest from all uphill sources.
The second analysis method is based on the Green-Ampt model of infiltration (Rawls, et al., 1983). Unlike the RCN approach, this model is spatially explicit and can simulate the effects of the location of future urban development. Because of this, it is more appropriate for larger basins. Rain falls on the landscape and, if not intercepted by vegetation or structures, either infiltrates or runs off. The factors that control these steps include rainfall intensity, the land cover, existing soil moisture, roughness of the land-vegetation surface, and slope. These data are measured for each named soil series and reported in the NRCS soil surveys. A computer model developed by Julien (et al., 1993) and implemented in the raster GIS program GRASS (Construction Engineers Research Laboratories, U.S. Army Corps of Engineers) is used to simulate both overland and channel flows.
The 25-year storm of 125mm is used as the test precipitation. It is useful to define the meaning of this storm designation. Historically, rainstorms that dropped 125mm of rain or more in the Camp Pendleton region occurred in about 4% of all the years of the measurement record. This implies that they have occurred four times in any given 100-year period (not once every 25 years). Thus, the probability of a 125mm rainstorm during any year is 4%, and such a storm is called the "25-year storm." (Note that .04 and 25 are recriprocals.) The rainfall distribution of the 25-year storm precipitation is shown in figure 33. To start the simulation, this rainstorm is distributed across the drainage basin. Then a proportion of the water is removed (by infiltration) as per the RCN or the Green-Ampt model, and the remainder is routed across the landscape to the stream and then downstream past the point of measurement where discharge is measured and plotted.
Flood hydrographs for the 25-year storm and soils with medium levels of antecedent moisture were calculated for 14 of the third-order basins by the RCN model, and then analyzed statistically to extend the analysis to other third-order basins of the study area. A statistical approach to the generation of storm hydrographs is taken because the simple regression equation can be applied to any other third-order basin without following the full-scale NRCS method. The statistical relationships between change of RCN and the changes of the ratios of peak discharge to bankfull discharge and total storm discharge to bankfull discharge were excellent predictive models. As shown in figure 34, they can be used with great confidence to estimate the flood flow effects of land-use change on any of the unanalyzed third-order drainage basins.
Peak discharge is the highest discharge recorded during a flood. Total discharge is the total volume of stormwater discharge over the entire hydrograph. Bankfull discharge is the discharge at bankfull flow, which is the 1.5-year flood, or the flow that has a 67% chance of being exceeded in any given year. Bankfull flow is a useful baseline discharge that is often used as a denominator to standardize hydrologic variables across basins of vastly different sizes so statistical relationships such as these can be calculated.
Flood hydrographs for the 25-year storm and soils with 90% saturation were calculated for the five large stream basins by the Julien (et al., 1993) model. Figures 35 through 39 show the simulated hydrographs of 1990+ land cover conditions and how they compare with hydrographs generated by the different land cover pattern predicted by the Plans Build-Out alternative. The hydrographs were simulated as measured in the stream channel near the river mouth, so changes in total discharge are estimates of how much additional water is lost from the terrestrial system, including Camp Pendleton, and not retained as groundwater.
According to the model simulations, both peak discharge and total discharge will increase for the San Juan, Santa Margarita, and San Luis Rey basins but will remain the same or decline for the San Onofre and San Mateo basins. The changes will be most extreme for the Santa Margarita River. There, peak discharge will nearly double and total discharge will increase by about 50%. The bulk of development in the Camp Pendleton region will take place in the Santa Margarita drainage basin, so the greatest changes in hydrological regime will occur there.
The model that simulates these hydrographs uses seven variables: a coefficient that describes the roughness of the surface over which water flow (Manning's n), the amount of rainfall interception by vegetation and other surfaces, the amount of rainfall stored in vegetation and above-ground structures, the moisture already in the soil, hydraulic conductivity of the soil, soil porosity, and the suction pressure at the wetting front as water moves down the soil column. In the simulations, the first three variables plus the changed extent of impermeable surface were affected by land use conversion, and the changes in the different river basins explain the mechanisms that cause different changes in the hydrographs.
The area of impermeable surface increased in a manner proportional to the area of development in each watershed, and rainfall that was not intercepted or infiltrated contributed to the volume of the flood hydrograph. The velocity of runoff was then controlled by the slope of the landscape and the roughness of the land cover. The roughness coefficient decreased in all drainage basins. The Santa Margarita River basin was predicted to be the most seriously affected by the development of buildings, paved roads and parking lots, and grasslands. The magnitude of roughness decrease in each of the watersheds was: 12.9% decrease in the Santa Margarita, 4.3% in the San Juan, 2.6% in the San Luis Rey, 1.2% in the San Mateo, and 0.3% in the San Onofre.
The relative changes in the hydrographs also follow this rank, suggesting that after the changes of runoff volume, changes in surface roughness control much of the change in flood characteristics. The changes in interception were not great (less than 1%), and did not contribute significantly to changed hydrographs. Likewise, the storage term was not strongly affected by land cover conversion.
These predicted changes of the two flood characteristics have profound implications for MCB Camp Pendleton. The airfield is vulnerable to increased flooding because it is located in the flood plain of the Santa Margarita River. It was badly damaged by the flood of January 1993, and the threat of increased peak discharge would put it at greater risk in the future. Flood protection for the airfield and adjacent facilities is in preliminary planning and a levee is the likely preferred alternative. Any long-term solution to mitigate flood risk must take into account the increased discharge and consequently higher flood elevations caused by future upstream land conversion.
As noted earlier, Camp Pendleton's water supply is taken totally from on-base groundwater supplies. For a single 25-year storm, total discharge form the five river basins will increase by 48% between 1990+ and Plans Build-Out. Approximately 140 million cubic meters of additional water will be lost to the region's aquifers because it flows to the ocean so rapidly that it cannot infiltrate into the ground. The spreader basins in the flood plain near Lake O'Neill are designed to capture a significant amount of the Santa Margarita River's discharge under most conditions. However, they were not able to perform adequately during the 1993 flood because they were saturated and substantially filled by relatively impermeable silts transported by flood waters. An increase in the amount of water flowing through and out of Camp Pendleton necessarily means that there will be less water for aquifer recharge and, over the long term, less water available for use.