Hydrologic Analysis of Watersheds West of Zacatecas, Zacatecas, Mexico

Hydrologic Analysis of Watersheds West of Zacatecas, Zacatecas, Mexico Jaron Brown, Joshua Draper, Derek Lounsbury April 18, 2007 1

Abstract The City of Zacatecas Mexico is experiencing rapid growth that is expected to continue for the foreseeable future. This change from agricultural or natural lands to urbanized areas will affect the runoff coming out from the areas. The change in runoff needs to be known in order to appropriately design and build civil structures such as culverts and bridges. This study analyzes three watersheds in the western zone of Zacatecas to model current and future runoff conditions. HEC-1 and HEC-HMS were each employed to analyze the runoff change; both predicted an increase in runoff as a result of development changing land use. Runoff calculations for current and future conditions by HEC-1 were 33 and 36 m 3 /s respectively; current and future runoff calculated by HEC-HMS was 56 and 63 m 3 /s respectively. Knowing that the runoff will change significantly over the next few decades, civil engineers can build structures now that will be capable of safely and effectively passing runoff from storm events for years to come. 2

Contents List of Tables List of Figures i i 1 Introduction 1 1.1 Background.................................... 1 1.2 Objectives..................................... 1 2 Methods and Data 2 2.1 Watershed Delineation.............................. 2 2.2 Soil and land use................................. 2 2.3 Precipitation................................... 5 2.4 Model Comparison................................ 5 3 Results 10 4 Discussion of Results 12 5 Conclusions 13 6 References 14 Appendix 14 A Data 15 List of Tables 1 Basin Composite Curve Numbers......................... 5 List of Figures 1 The DEM from INEGI had an error that caused an unnatural corner in the delineated watershed................................ 3 2 The watershed delineated from the USGS DEM is much more reasonable. Basins (from top to bottom): La Pimienta, La Escondida, Southern...... 4 3 The land use map does not account for the city extents which can be seen in light gray...................................... 6 4 Plot of IDF curves received from UAZ...................... 7 5 Temporal rainfall distribution for Zacatecas................... 7 i

6 Hydrographs for present conditions. The higher peak came from HEC-HMS and the lower peak came from HEC-1...................... 10 7 Hydrographs for future conditions. The higher peak came from HEC-HMS and the lower peak came from HEC-1...................... 11 ii

1 Introduction Zacatecas is a rapidly growing city in north-central Mexico that is home to the Universidad Autónoma de Zacatecas (UAZ) which has participated for the last three years in a collaborative program with BYU to develop hydrologic and hydraulic modeling experience using the Watershed Modeling System (WMS). This year, one of the objectives was to analyze the effect of planned urbanization in order to perform flood runoff analysis for the city. The results can then be used to decide what flood prevention measures may be necessary. 1.1 Background The city of Zacatecas is the capital of the state of Zacatecas, Mexico and is home to approximately 120,000 inhabitants. [1] Recently, the city has experienced rapid growth as it has become a political, economic, and social center of great importance due to its geographic location. The city is also a world renowned tourist area and contains the state s principal educational institutions such as UAZ and others. All of these factors have attracted people from the surrounding cities and other areas to immigrate to Zacatecas. The large influx of people has resulted in an increase in urban and agricultural land uses that has changed the characteristics of the surrounding watersheds including reduced infiltration, increased erosion rates, and climatic changes. [2] 1.2 Objectives In order to quantify the effect that the current and future expansion will have on the hydrologic resources of Zacatecas, accurate hydrologic models are needed in order to assess and predict changes in runoff in the various watersheds. The objective of this project is to use WMS to delineate the watersheds and generate initial models in HEC-1 and HMS which produce reasonable results for the area. In later years, these models can be further refined, calibrated, and validated so they can then be used as predictive models. 1

2 Methods and Data In order to analyze the flood runoff, two HEC-1 models of the same area were built. The first model simulated the current land use in Zacatecas. The second model simulated the land use after the planned urbanization had taken place. HEC-1 was chosen because it is designed to simulate single storm events and is the current industry standard for flood runoff analysis. The three kinds of data WMS needs in order to complete the models are geometric data for delineation, basin hydrologic characteristics, and meteorological data. A digital elevation model (DEM) was used to delineate the basins. Both models were developed using the SCS curve number loss method and the Clark unit hydrograph method. A rainfall temporal distribution was generated for the area and the precipitation input as basin averages. After the HEC-1 analyses were finished, pre- and post-development conditions were also modeled in HMS using the ModClark method to investigate the difference between the lumped parameter model (HEC-1) and the distributed model (HMS). 2.1 Watershed Delineation The initial delineation process was accomplished using a DEM obtained from a Mexican government body called INEGI (National Institute of Statistics, Geography and Informatics). [3] Unfortunately, the DEM had an error that caused WMS to delineate the watershed with an unnatural corner as can be seen in Figure 1 on page 3. Later, a DEM obtained from the USGS yielded a much more reasonable delineation that was then used for all the modeling and can be seen in Figure 2 on page 4. The advantage of using a DEM for delineation is that the basin geometry can be automatically determined from the elevation information. Also, important parameters such as slope, time of concentration, and elevation-storage curves can be automatically calculated. The disadvantage is that DEMs do not usually capture linear or man-made features, so the delineated watershed may not represent the real watershed accurately. Even after delineating the watershed with the good DEM, some streams had to be manually adjusted so that they lined up with the rivers on the topographic map of the area. Three outlets were specified for this study. One was downstream from the town of La Pimienta and resulted in the north (top) watershed in Figure 2. The second outlet was just downstream of the town of La Escondida. La Escondida is the middle basin. The third, southernmost outlet split the La Escondida watershed into two sub-basins to account for different topography in each area. 2.2 Soil and land use Once a reasonable delineation was obtained, a composite curve number was needed to run HEC-1. The Watershed Modeling System calculates a composite curve number using land use and soil type coverages or shapefiles. On each coverage or shapefile, polygons are defined which are given attributes that describe the land uses and soil types. Each soil type polygon is assigned to one of four hydrologic groups: A, B, C, and D. Each land use has four curve 2

Figure 1: The DEM from INEGI had an error that caused an unnatural corner in the delineated watershed. 3

Figure 2: The watershed delineated from the USGS DEM is much more reasonable. Basins (from top to bottom): La Pimienta, La Escondida, Southern. 4

Table 1: Basin Composite Curve Numbers. Basin Curve Number La Pimienta 71.8 La Escondida 70.9 Southern 57.1 numbers associated with it, one for each hydrologic group. WMS then determines the curve number for each land use polygon by overlaying the two coverages and determining which soil type is under each land use polygon. The composite curve number for the basin is then found by taking an average of the individual curve numbers weighted by area. For much of the United States, this process can be automated using shapefiles which contain all the necessary information, but such shapefiles did not exist for Zacatecas. The only land use and soil type information available came in the form of printed maps that were scanned, imported into WMS, and digitized into polygons. After generating the soil type and land use polygons, a CAD drawing of the city was superimposed over the maps and it was discovered that the land use map did not account for the city; therefore, many of the land use polygons had to be redrawn or adjusted. Figure 3 on page 6 shows the CAD file and land use polygons superimposed on the land use map. After correcting all the land use polygons to reflect the current extents of the city, the composite curve numbers for each basin were calculated and are listed in Table 1. The northern basins curve numbers of about 70 initially seemed too high, but were deemed reasonable after a site visit. The southernmost basin s curve number of 57.1 also agreed with observed conditions. 2.3 Precipitation UAZ sent intensity duration frequency (IDF) precipitation data for Zacatecas which is charted in Figure 4 on page 7. Since HEC-1 only models single storm events, the IDF curves were transformed into a normalized temporal rainfall distribution using the SCS design storm method. The resulting curve is shown in Figure 5 on page 7. The distribution was derived using data for a 10 year recurrence interval, but should apply to any storm in the area. The models were then run with storm precipitation depths of 75 mm (3 inches) because that represents an average 10 year storm event. 2.4 Model Comparison After the results of the HEC-1 analysis were delivered to UAZ, a comparison with HEC- HMS using the ModClark unit hydrograph method with gridded SCS curve numbers was performed. For comparison purposes, the La Pimienta basin was not considered and the two southern basins were combined into a single watershed. This was done because WMS 5

Figure 3: The land use map does not account for the city extents which can be seen in light gray. 6

Figure 4: Plot of IDF curves received from UAZ. Figure 5: Temporal rainfall distribution for Zacatecas. 7

is currently only capable of setting up the input files for HEC-HMS for one basin. The same land use and soil type coverages were used with the same hyetograph and precipitation depth. Developing the HEC-HMS models uncovered an important anomaly in version 8.0 of WMS which was the version used in this project. For HEC-1, if the temporal distribution of rainfall does not total 1 unit and a precipitation depth is specified, WMS will normalize the distribution and then multiply by the depth. However, for HEC-HMS, WMS does not normalize the temporal distribution. It uses the total rainfall from the distribution and multiplies by the given depth. The result is that a temporal distribution that describes a large amount of rainfall multiplied by a large average depth produces numbers that are too big to be written the precipitation file. For example, the Zacatecas distribution was produced using 120 mm of rain and input into WMS without normalizing it. The basin average depth of 75 mm was entered and the HMS file saved. When viewed in HMS, the precipitation value was zero where the peak was suppposed to be because the numbers were too high to be written to the file. After normalizing the temporal distribution, everything worked as expected. To summarize the HEC-1 model parameters, the ADOT (desert/mountains) method was used for calculating time of concentration. This equation is defined as: T c = 2.4A 0.1 L 0.25 L c a 0.25 S 0.2 (1) where T c = time of concentration, hrs A = basin area, km 2 L = length of the longest flow path, m L c a = length along the main channel from the outlet to the point opposite the centroid of the basin area, m S = slope of the channel from outlet to furthest point in the watershed,m/m This equation was used because the area around Zacatecas is very similar to that of the State of Arizona. Nopal (prickly pear) cactus, sagebrush, and sandy/rocky soil are prominent features of the terrain in both locations, so it seemed to be a reasonable fit for our model of Zacatecas. For a loss method, the SCS Curve Number method was selected. As has been previously described, composite curve numbers were used for each basin or sub-basin. Precipitation data was defined as a basin average of 75 mm and a given temporal distribution. Muskingham- Cunge routing was chosen for its simplicity and ease of use. Manning s n value was.03, and the channel shape was assumed to be roughly trapezoidal with a bottom width of 4 meters and side slopes of 5:1. The HEC-HMS model parameters were as similar to the HEC-1 runs as possible. Again the T c was calculated with the ADOT equation, and the same precipitation data (same basin average and temporal distribution) were used. The unit hydrograph method differed however. For HEC-HMS, the ModClark method was used. Also, the models were different in the way the CN values were used. While HEC-1 computes 8

a composite curve number for the basin area, HEC-HMS uses a grid (in our case it was a 40 X 40 grid), and a different curve number was computed for each grid cell individually. Then, the grid cells are used one by one for routing and runoff calculations. The difference in methods for handling CN values is what makes the models different and where the different results come from. 9

3 Results The results of a HEC-1 model of the La Pemienta watershed indicated a peak flow of approximately 14 cms. No urban growth is planned within the boundaries of the La Pimienta watershed; ergo, flow rates under future conditions of Zacatecas remained the same. Using the HEC-1 and HEC-HMS models to calculate the runoff rates of the La Escondida watershed, hydrographs illustrating flow rates at the outlet of the La Escondida watershed under current land use conditions and after projected urban growth were created as seen in Figure 6 on page 10 and Figure 7 on page 11, respectively. The peak flows determined using the HEC-1 model were approximately 33 cms for the present conditions and 36 cms for the expected future conditions. The peak flows for the present and future conditions determined using HEC-HMS were approximately 56 cms and 63 cms, respectively. Both modeling methods calculated a 9% to 13% increase in runoff under future land use conditions. The value of the peak runoff rates determined using the HEC-HMS model for the present and anticipated future conditions were approximately 70% to 75% greater than the peak values determined using the HEC-1 model. Figure 6: Hydrographs for present conditions. The higher peak came from HEC-HMS and the lower peak came from HEC-1. 10

Figure 7: Hydrographs for future conditions. The higher peak came from HEC-HMS and the lower peak came from HEC-1. 11

4 Discussion of Results HEC-HMS consistently gave much higher peak flows than HEC-1. The only difference in the HEC-1 and HEC-HMS models that were used is the method of accounting for spatial variability in the data, so it must account for the difference in flows. HEC-1 with the Clark unit hydrograph method and SCS curve number loss method is a lumped parameter model meaning that spatial heterogeneity is treated by using weighted averages. HEC-HMS with the ModClark method and gridded SCS curve numbers divides the watershed into a grid of tiny subbasins each with its own parameters and routes the results from each grid cell to produce the final hydrograph. These models have not yet been perfected. Due to the fact that the scanned maps did not cover the entire watershed and educated guesses were made as to the true land use and soil type for areas not covered, results are probably not truly representative of the runoff found in real life. Also, since the land use maps provided by colleagues in Mexico did not coincide with the AutoCAD map of the city, placing large amounts of confidence in the map in other areas would be a mistake. Furthermore, since there are no gauges on the streams to give real-time estimates on the flowrates, it is impossible to say whether the results found in this study are even in the ballpark with what actually happens during a storm event like the one modeled. Future researchers working on this project may wish to verify the land use map or look for a more current one as well as find land use and soil type maps for the rest of the watershed. The models could also be calibrated and validated so they could be deemed reliable. This would require gauging the stream and searching for historical records of stream flows in La Escondida and La Pimienta. 12

5 Conclusions Despite the model deficiencies mentioned above, it is necessary to remember that the objective of this analysis is to quantify the relative change in runoff between present and future conditions. Both HEC-1 and HEC-HMS predicted about a 13% increase in the runoff due to urban growth over the next few decades. If planners really want to know if existing structures downstream (culverts and bridges) are going to be able to pass the increased flows, they need only gauge the streams during storm events and predict a 13% or so increase in whatever flow rate they find in the field. The data found here will be useful in that respect. 13

6 References References [1] http://en.wikipedia.org/wiki/zacatecas. website, April 2007. [2] Oscar Antonio Dzul García. Cooperación universitaria internacional 2007 universidad autónoma de zacatecas brigham young university. January 2007. [3] Saul Gutierrez. Implementation of digital information to design hydrologic models in mexico. Master s project, Brigham Young University, December 2006. 14

Appendix A Data Attached to the hard copy of this report is a CD that contains all of the data necessary to continue work on this project. The README.txt file from the CD is reproduced below. This CD contains the project and supporting files for the Zona Poniente project. All required files to continue the work are included. Please see the file listing below for a description of the contents. In order to interpret the file listing below, an ending "/" denotes a directory. Coordenada outlet cuenca.txt Original outlet coordinates coordinates.xls Updated outlet coordinates Descripcion.pdf Original project description recieved from Mexico HEC-1/ Contains the project used to run the HEC-1 simulations for UAZ HMS/ Contains the projects used for comparing HEC-1 to HEC-HMS images/ Project images Presentations/ Presentations given in Mexico and at BYU project plan.rtf README.txt ZonaPonCNtable.txt Mapping table for WMS Zona_Poniente_Hydrologic_Modeling_Report.pdf Final report Zona-Poniente-IDF.xls Original IDF data and generate temporal distribution ZonaPoniente.wmv Google earth video of the watershed area./hec-1: usgs-delineated.bsn usgs-delineated.dwg usgs-delineated.fac usgs-delineated.fdr usgs-delineated.gdm usgs-delineated.ini usgs-delineated.lsf usgs-delineated.map usgs-delineated.sto usgs-delineated.tre usgs-delineated.wpr zonaponiente-10yr.xys Project used for analysis in Zacatecas This file is the temporal distribution used. 15

./HMS: Projects used for BYU class CE En 531 basinstates/ cn_report.txt Future/ Project representing future conditions good-landuse.map hydrographs.xls optimizer/ Present/ PUNCH Run_1.log Run_3.log southfut.access southfut.basin southfut.control southfut.dss southfut.dss.msg southfut.gage southfut.grid southfut.hms southfut.log southfut.map southfut.met southfut.mod southfut.out southfut.run southfuture.hc1 southfuture.out southfuture.sol southpre4.access southpre4.basin southpre4.control southpre4.dss southpre4.dss.msg southpre4.gage southpre4.grid southpre4.hms southpre4.log southpre4.map This is a land use coverage with correct polygons. Excel file that compares the pre and post condition hydrographs Project representing present conditions southfut is the future conditions of the southern basin HEC-1 files for future conditions of southern basin southpre4 is the current conditions of the southern basin 16

southpre4.met southpre4.mod southpre4.out southpre4.run southpresent.hc1 HEC-1 files for present conditions on southern basin southpresent.out southpresent.sol Thumbs.db uso de suelo zac2.jpw uso de suelo zac2.tiff ZonaPonCNtable.txt./HMS/basinStates:./HMS/Future: zonaponientefuturo.2dg zonaponientefuturo.bsn zonaponientefuturo.dwg zonaponientefuturo.fac zonaponientefuturo.fdr zonaponientefuturo.gdat zonaponientefuturo.gdm zonaponientefuturo.ini zonaponientefuturo.lsf zonaponientefuturo.map zonaponientefuturo.sto zonaponientefuturo.tre zonaponientefuturo.wpr./hms/optimizer:./hms/present: zonaponientepresente.2dg zonaponientepresente.bsn zonaponientepresente.dwg zonaponientepresente.fac zonaponientepresente.fdr zonaponientepresente.gdat zonaponientepresente.gdm zonaponientepresente.ini zonaponientepresente.lsf zonaponientepresente.map 17

zonaponientepresente.sto zonaponientepresente.tre zonaponientepresente.wpr./images: edafologia Zac2.tiff Edafologica Zac.jgw Edafologica Zac.jpg Edafologica Zac.jpw Edafologica Zac.tiff Geologica Zac.tiff Map words.rtf Simbologia edafologia2.tif Simbologia edafologia.tif Simbologia geologica.tif Simbologia uso de suelo.tif Topografia Zac.jpg Topografia Zac.jpw topografico Zac2.tiff uso de suelo zac2.jpg uso de suelo zac2.jpw uso de suelo zac2.tiff Uso suelo Zac.jpg Uso suelo Zac.jpw ZonaPoniente.jpg./Presentations: gobernadora.ppt landusepoly.png presentacionenespaol.ppt Thumbs.db zac-area of interest.jpg Zacatecas-mexico.jpg zona poniente english.ppt Southern area soil type map Description of mapping table entries Soil type map legend Soil type map legend Geology map legend Land use map legend Topologic map Southern land use map Northern land use map Very high resolution image of watershed area. A presentation of the project geared toward the governer of Zacatecas. Preliminary presentation in spanish 18