 Jemez y Sangre Water Planning Council - Water Budgets
Water Budget
The water budget for both surface and groundwater that is presented in this section is based on the Duke water supply study (2001), which provides a more detailed explanation of water budget values and the uncertainty involved in deriving these estimates. The water budget estimates originally prepared by Duke were subsequently updated by the JySWPC. This section discusses the terms and methodology used in the Duke water supply study and then summarizes the surface water and groundwater supply components for each sub-basin, revised with additional information as applicable.
Terms and Methodology
A water budget evaluates the hydrologic balance of an area through describing its inflow, outflow, and storage characteristics. Figure 21 is a schematic diagram illustrating the water budget components for the Jemez y Sangre region.
Surface Water Budget Terms and Methodology
Table 21 summarizes the surface water budget for the planning region.This budget is explained in more detail in Sections 6.2.2 through 6.2.11.Terminology used for describing the surface water budget is defined below.
Surface-water inflow is the amount of water that annually enters the sub-basin as surface runoff. The exact method used to evaluate this component varies depending on the characteristics of the sub-basin for which it is being determined. In the Santa Cruz, Pojoaque-Nambe, Tesuque, and Santa Fe River Sub-Basins, surface-water inflow refers to the amount of runoff occurring at the mountain-front, which, in this study, is defined as the lowest elevation at which crystalline rocks outcrop. In the Velarde Sub-Basin, surface-water inflow is defined as the combined flow entering the sub-basin from the Rio Grande and mountain-front runoff. In the Santa Clara and Los Alamos Sub-Basins, inflow represents the runoff generated near the eastern-most extent of lava flows on the Pajarito Plateau. Surface-water inflows to the Caja del Rio and North Galisteo Creek Sub-Basins were estimated for the entire surface area of each. In the South Galisteo Creek Sub-Basin, surface-water inflow was assumed to equal the runoff generated above an elevation of 6,500 ft msl.
Wherever possible, stream-gage data were used to estimate surface-water inflow.In most sub-basins, however, stream flow gaging stations do not coincide with the locations (identified above) where inflow is determined. Thus,most of the inflow estimates were developed using the elevation-area-yield approach (Reiland, 1975).
Stream gain is the amount of water that flows into the stream from springs or seeps from an aquifer, while stream loss is the amount of water that seeps out of the stream and recharges the aquifer. Data for this gain or loss were not well quantified, and consequently Duke estimated this amount as the residual from all of the other budget components. In a few sub-basins the gain or loss was estimated directly if (1) stream-gage data were available, (2) measured spring flow data were available, or(3) estimates of stream-aquifer exchange quantities were available from separate sources.
Total evapotranspiration in a sub-basin represents combined FWS evaporation and ET from riparian areas where the water table is assumed to be, on average, quite shallow(less than 20 feet below ground surface). The FWS evaporation outflows from streams, canals and reservoirs were developed by multiplying the surface areas of perennial water bodies by an average evaporation rate of 45 inches per year (in/yr). Estimates of ET losses from riparian corridors were developed by multiplying riparian surface areas by representative PET rates. Land sat imagery provided by Santa Fe County for land use in1992 was used to delineate the riparian areas.
A surface-water diversion is the amount of water removed from a stream for human use (e.g., irrigation or drinking water). Irrigation diversions were estimated by multiplying an irrigated acreage value by an irrigation application rate that varied with each sub-basin. The surface water budget shows the amount of water diverted from the stream as opposed to the amount of water actually consumed. The amount of water that is not consumed through crop ET or other incidental depletions either returns to the stream system or recharges groundwater.
Water that returns to the system is called return flow water. Return flows consist of (1) irrigation applications that initially seep into the ground and recharge the aquifer but eventually return to a natural watercourse through springs and seeps and (2) surface flow in canals and acéquias that returns to a watercourse.
Several sources were considered for estimating irrigated acreage in the various sub-basins. These included (1) the planning office of Rio Arriba County, (2) Wilson and Lucero (1997), (3) 1992 Land sat imagery, and (4) hydrographic surveys for various parts of the region. The Duke study compared the irrigated acreages determined by each source and revealed large discrepancies between the estimates. This uncertainty in actual water use poses a problem for water planning. Without a complete adjudication of the region, this problem will remain. The estimates of irrigated acreage determined from the three methods were presented in Section 5 (Tables 13 and 14).
Surface water diversions for municipal use in the Santa Fe River Sub-Basin were estimated using records of measured flows. The average of annual Santa Fe River diversions for urban use between 1990 and 1999 was used to develop the budget for this sub-basin.
Groundwater Budget Terms and Methodology
Table 22 summarizes the groundwater budget for the planning region. Sections6.2.2 through 6.2.11 explain this budget in greater detail. The terms and methodology used to estimate groundwater budget components are described below.
In a groundwater budget, the total inflow and outflow components are not equal when water levels are either rising or falling. If outflow is greater than the inflow,water levels will lower in the aquifer and the volume of water in storage will decrease. Where the change in storage is negative, water levels in the sub-basin are dropping, and where the value is positive, water levels are rising. It is possible to have water levels dropping in one location and rising in another within the same sub-basin, as is the case in the Santa-Fe River Sub-Basin. Recharge from the effluent from the wastewater treatment plant is causing water levels to rise in the reach from the Santa Fe Airport to the Rio Grande, yet water levels in the vicinity of the City have dropped hundreds of feet over the last 50 years.
The inflow components of the groundwater budget consist primarily of various mechanisms of recharging an aquifer and the inflow that occurs from one sub-basin to another. Recharge from stream losses and mountain-front recharge are the two natural mechanisms for recharge (Figure 21). Area recharge from precipitation in areas other than mountain fronts is considered by many researchers to be very small and assumed to be zero in water budgets for the region.
Recharge from stream losses is equivalent to stream seepage, an outflow component of the surface water budget. In areas where the aquifer water level is below the stream level, the stream loses water and the aquifer is recharged. The amount of recharge depends on the flow in the stream,the amount of clay on the bottom of the stream, and the type of geologic formation that separates the stream and the aquifer. As stated earlier,without stream gaging, this amount is estimated as a residual in the water budgets of most sub-basins in the region.
Mountain-front recharge consists of sub-surface flow across the interface between basin sediments and the igneous rocks that are found on the eastern and western margins of the Española Basin. Because mountain-front recharge has not been measured directly, it, like groundwater/surface water exchange,is considered one of the most uncertain water budget components. Mountain-front recharge was estimated as the remainder of precipitation minus evaporation and runoff. Using mass balance techniques involving annual volumes of precipitation, evaporation, and surface runoff from the mountains, Duke developed separate estimates of mountain-front recharge for each of the sub-basins in the Jemez y Sangre Water Planning Region. To produce the estimates, the mountain front along the eastern side of the planning region was delineated as the contact line between the crystalline rocks of the Sangre de Cristo Range and sediments of the Santa Fe Group. The mountain front along the western side of the planning region was delineated as the contact line between the volcanic rocks of the Jemez Mountains and the sediments of the Santa Fe Group.
Average precipitation in the mountains was estimated using the precipitation map developed by Wasiolek (1995), and the PET map prepared by Tuan et al. (1969) was used to estimate the PET for each sub-basin. Representative values for both precipitation and PET volumes were developed for the mountainous parts of each sub-basin via weighting of areas between contours of these variables(i.e., the area between contours was estimated and multiplied by the average value between contours).
The surface runoff volume associated with mountainous areas in each sub-basin was estimated using either the area-elevation-yield approach of Reiland (1975)or gaging station data. A total of about 27.5 cubic feet per second (cfs)(19,940 afy) is estimated to recharge the regional aquifer system via mountain-front sources in the Sangre de Cristo Mountains. The estimated subsurface inflow from the mountain front along the Jemez Mountains is about 10.5 cfs (7,600 afy).
Flow from adjacent sub-basins is the water that flows underground across sub-basin boundaries. The flow into one basin is equivalent to the flow out of another basin. The methodology for calculating the flow is described below.
Return flow to groundwater was estimated from the Wilson and Lucero (1997)estimated rates of return flow for diversions. Return flow from septic tanks was estimated as 50 percent of the amount diverted from domestic wells. The return flow values provided in Table 22 differ from the values presented by Duke (2001, Table 5-9) because the estimates of diversions from domestic wells and other metered wells was modified due to revised population estimates. The return flow for municipalities varied, based on data from Wilson and Lucero (1997). For the City of Española,the return flow was 80 percent of the diversion, while Los Alamos’return flow was estimated by Duke (2001) to be 4 percent of the diversion,largely because much of the Los Alamos water is consumed in the industrial processes at LANL and in reuse through turf application. However, diversion records for 2001 and 2002 indicate that return flow in Los Alamos maybe closer to 30 percent. In Santa Fe, Duke reported the return flow to groundwater from effluent (2,170 acre-feet) as stream loss; Table 22 incorporates this into return flow to groundwater. This return flow occurs between the wastewater treatment plant and La Cienega. The return flow from Theodora community system is estimated as 50 percent of the diversion.The return flows from irrigation are included in the estimates for Velarde, Pojoaque-Nambe, and Santa Fe River Sub-Basins.
The groundwater outflow components consist of both natural and man-induced mechanisms .Groundwater discharges naturally to the Rio Grande and its tributaries where the water level in the aquifer is higher than the stream. Groundwater can also flow out of one sub-basin into another sub-basin. Other natural processes for groundwater loss include evapotranspiration from a shallow water table and discharges to springs. Groundwater pumping through wells is a man-induced groundwater discharge.
Groundwater discharge across sub-basin boundaries is categorized as the outflow from sub-basin. Considerably large groundwater flows occur between basins. Estimates of inter basin flow were developed using Darcy’s Law and appropriate hydrologic parameters. These initial values were then adjusted to account for other influences on the transfer of water such as possible vertical gradients and deeper aquifer thickness in some locales. In locations where the ground elevation intersects the water level elevation, groundwater discharges to springs or seeps and flows to the Rio Grande or its tributaries. The amount of groundwater discharge to surface water is equal to the surface water inflow discussed in Section 6.2.1.1 and is estimated as the residual of the surface water budget, except where specific data are available.
Groundwater discharged through ET occurs when the roots of trees or other vegetation tap the aquifer and consume water directly from the aquifer. Groundwater discharge to ET was estimated for areas with a depth to groundwater of 20 feet or less. This estimate was not intended to overlap with other estimates of water loss to the atmosphere such as losses due to ET by irrigated crops and ET riparian vegetation. Consequently, this discharge component was ascribed to locales away from known irrigated and riparian areas. A depth to groundwater of 20 feet or less was chosen as the depth at which losses to ET could occur, based on the fact that phreatophyte trees typically have rooting depths of about 33 feet (Bouwer, 1978) and phreatophyte shrubs commonly root to a depth of 10 feet.
A depth-to-groundwater map was used to estimate the acreage where the depth to the water table was less than 20 feet, and this was multiplied by an ET rate estimated by subtracting the mean annual precipitation for the area from the sub-basin PET rate. A total of about 13,650 afy was estimated for groundwater discharged through ET for the entire planning region.
Pumping of groundwater in the planning region is mainly for municipal and domestic uses. A small amount of groundwater (about 730 afy) is diverted for irrigation use. Estimates of diversion for this component were revised from Duke (2001) based on updated population estimates, a compilation of metered wells provided by Wilson and Lucero (1997), and the municipal pumping records for Los Alamos, Española and the City of Santa Fe.
Community wells, including municipal wells, are individually metered and are required to report usage to the OSE. Annual production from municipal wells is provided in Duke (2001); the quantity diverted from community wells is reported by Wilson and Lucero (1992, 1997).
The amount of water diverted from domestic wells was estimated indirectly, since domestic wells are generally not metered. The population estimate for the year 2000 was multiplied by 0.15 acre-foot per person per year (approximately 134 gpcd) for each sub-basin (except Santa Fe) to obtain the total amount of water needed to support the population. The amount of measured (metered) usage was subtracted from this amount to obtain the residual quantity that is assumed to be met through domestic wells. The 0.15 acre-foot per person includes all nonagricultural uses of water in each sub-basin. The domestic wells in sub-basins without municipal systems are likely to serve businesses such as gas stations, restaurants, etc. in addition to the domestic usage.
For the Santa Fe River Sub-Basin, the rate of water use per person on the City water system is 0.183 acre-foot per person when no drought restrictions are in place. (This value reflects a reduction in water usage from 0.23 acre-foot per person per year, which was the usage rate prior to the implementation of the conservation ordinance that restricts watering from 10 a.m. until 4 p.m. during the summer months.) To obtain the amount of water derived from domestic wells in the Santa Fe River Sub-Basin, the population for 2000 was multiplied by 0.183 acre-foot per person per year (approximately 163 gpcd) and the amount of metered usage was subtracted to obtain a volume for the demand not supplied by a water system. This residual was then divided by 0.183 acre-foot per person to obtain the population supplied by individual wells. The population using individual wells was multiplied by 0.096 acre-foot per person to obtain a reasonable estimate for the amount of water diverted from domestic wells for domestic use only.
The groundwater budgets are highly uncertain. Many of the components have been developed using the principle of mass balance, in which the summed components of groundwater inflow are expected to equal the sum of outflow components and rates of change in groundwater storage. However, the estimated budget components should be tested to determine if they are mathematically consistent with measured hydraulic heads and/or measured stream/aquifer exchanges in a three-dimensional environment. To determine such consistency, numerical models that are capable of simulating three-dimensional groundwater flow are most useful. If a numerical model is developed with enough detail to facilitate the quantification of various groundwater flow processes on a relatively local level, it should ultimately provide much better estimates of budget components such as inter basin subsurface flow, vertical groundwater movement, and stream-aquifer interaction.
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