0080-B1

Water Dynamics on the Slopes of the Sierra Madre Oriental, Mexico

Jose Santos Flores Laureano^[1], Anthony G. Price, Jose de Jesus Navar Chaidez and Rorke B. Bryan

ABSTRACT

The state of Nuevo Leon, Mexico, is situated in a semiarid region. The Sierra Madre Oriental (SMO) is a mountain range that traverses the state and plays an important role in its hydrological regime. However, very little is known about the dynamics of water on the slopes of the SMO. The objectives of this study were: (i) to define water pathways and fluxes on the slopes of some typical sites of the SMO, (ii) to identify the factors that control water motion, and (iii) to assess the influence that vegetation type has on water pathways.

Five representative sites were selected where throughfall, overland flow, subsurface flow, and soil water content change were measured. Two approaches were evaluated: natural rainfall and irrigation. Overland flow only occurred in plot 1 located on a south-facing slope. Subsurface flow was absent or small in the forested plots (north-facing slopes). The dominant process in these slopes is water infiltration which, coupled with a rapid percolation of water through macropores into highly fractured bedrock, assured a rapid transmission of water to the bedrock. The observed hydrologic response differed from that expected for semiarid and temperate regions.

1. Introduction

The state of Nuevo Leon, located in northeastern Mexico, has large areas with problems of water scarcity. The majority of the rivers in Nuevo Leon originate in the Sierra Madre Oriental (SMO) mountain range and flow through the physiographic province of the Great Plains of North America to the neighbouring state of Tamaulipas where they join the Rio Bravo, which flows to the Gulf of Mexico (see Figure 1). Runoff generated within the SMO is extremely important for millions of inhabitants living in the plains region. The two principal water uses are: (i) urban consumption in the Monterrey Metropolitan Area (MMA) in Nuevo Leon; and (ii) agricultural use in the State of Tamaulipas. Due to the accelerated growth of the MMA two big water reservoirs were constructed during the past 18 years. As a consequence, the quantity of water received by Tamualipan farmers has been reduced causing an acute and complex legal and socioeconomic conflict between both Nuevo Leon and Tamaulipas.

Unfortunately, information concerning the characteristics of hydrological processes in the Sierra Madre Oriental is limited. The specific objectives of this study were: (i) to define water pathways and fluxes on the slopes of some typical sites of the SMO, (ii) to identify the factors that control water motion, (iii) to assess the influence that vegetation type has on water pathways. This information is extremely important because it allows a better understanding of the interactions among water, climate, soil, and vegetation, and consequently a better process of planning and decision making regarding the management of natural resources can be carried out.

2. Materials and methods

2.1 Description of the study area

The study was carried out within the Experimental Forest (EF) of the Universidad Autonoma de Nuevo Leon. The EF is located at 24°43’ N and 99°52’ W (see Figure 1). Marroquin (1987) described the geology of the EF as being composed of shales and limestones from the late Cretaceous together with recent sedimentary deposits. The soil orders present in the EF are Entisol, Inceptisol and Mollisol (Woerner 1990). The annual mean precipitation is 687 mm, and the annual mean temperature is 13.9 ºC. Marroquin (1987) classified the vegetation present in the EF into six types: pine forest, oak forest, canyon forest, oak-ash-cedar forest, stands of cedar, and shrub-chaparral.

2.2 Methodology

Five typical sites were selected where 5 plots were established. Plot 1 was located on a southeast-facing slope with a matorral (shrub) vegetation type. Plot 2 was located in a mixed stand of cedar and oak on a northeast-facing slope, while plots 3 and 4 were located within oak stands on northwest-facing slopes. Plot 5 was located under a mixed oak-pine stand on a northeast-facing slope

Two approaches were evaluated: natural rainfall and irrigation. The variables measured were gross precipitation, rainfall duration, rainfall intensity, throughfall, antecedent soil water content, evolution of soil water content during and after the irrigation experiments, overland flow and subsurface flow. Some geological and soil variables were also measured.

Within each study plot a subplot (8 x 4 m) was isolated in order to study runoff and soil water dynamics. At the bottom of each subplot a 4-m long trench was dug to the bedrock in order to install two troughs, one at the surface to intercept overland flow, while the other was positioned slightly lower than the level of the bedrock to collect subsurface flow. Due to the rockiness of site 1 only an overland flow collector was constructed.

In addition to a tipping bucket rain gauge, two collectors (638 cm² orifice area) placed in a forest clearing near each plot were used to measure gross precipitation. Throughfall was also measured by means of collectors (25 collectors were used in each plot).

For the irrigation experiments soil moisture content was measured by means of the gravimetric method and by using a Time Domain Reflectometer (TDR). The TDR was used only for 10 experiments. A single reading at the TDR probes was made few minutes before the beginning of the experiment. Two or three minutes after the beginning of the experiment the first TDR reading was taken; successive readings were taken every 5 minutes until the end of the experiment. After the end of the experiment readings were also taken every 5 minutes during the first 30 minutes, after that, the time interval between successive readings was increased. When runoff occurred, it was measured volumetrically, and transformed to equivalent depth. The rainfall intensities used ranged from 42 to 100 mm h^-1, which correspond to 1 year-1hour and 50 year-1 hour return interval respectively.

Figure 1. Geographic location of the study area. Map scales: a) 1 : 8 475 000, b) 1 : 876 500.

3. Results

3.1 Throughfall

Throughfall ranged from 85.2 % in plot 2 to 87.4 % in plot 4. The analysis of covariance, using rainfall as the covariate, showed that there are no significant statistical differences (P > F = 0.4481) in throughfall among plots. A simple linear regression using gross precipitation as the independent variable accounted for more than 99 % of the throughfall variation.

3.2 Runoff

Thirty-three rainfall events were measured. On the forested plots (plots 2 to 5) overland flow never occurred and subsurface runoff was very small, never exceeding 0.6 % of precipitation amount. On the other hand, only in very small rainfall events was there no overland flow at all on plot 1. The mean runoff coefficient in this plot was 8.5 % ranging from 0 to 47.8 %. Rainfall depth explained about 68 % of the variation of the runoff coefficient.

Thirty irrigation experiments were performed during the study. Overland flow only occurred on plot 1. The runoff coefficient ranged from 27.8 % to 62.0 %. Overland flow did not occur in the forested plots and subsurface flow only occurred under high antecedent soil water content; the runoff coefficient on plots 3, 4, and 5 never exceeded 1 % of irrigation depth. Plot 2 produced a more significant response since under large antecedent water content (35.5 % and 45.3 %) the runoff coefficient was 5.2 % and 9.2 %.

3.3 Soil Water Dynamics

During irrigation a similar pattern of soil water content evolution was observed in all runs (see Figure 2). Every curve was composed of three phases: (i) a wetting phase, (ii) a rapid drainage phase, and (iii) a slow drainage phase. During the initial wetting phase (from point A to point B in Figure 2), soil water content rises rapidly as a response to irrigation. In the later part of wetting (from point B to point C), some water is being lost out of the base of the soil, so the rate of increase of soil water content is reduced. Water loss was calculated by subtracting maximum (peak) water depth from total water depth.

After the cessation of irrigation, the soil water content decreased rapidly, which is due to water draining out of macropores. Subsequent drainage is interpreted as drainage out of smaller pores (micropores) in the soil. Water loss during the rapid drainage phase was calculated by subtracting soil water content when rapid drainage ceased (point D in Figure 2) from that when irrigation ended (point C in Figure 2). Similarly, water loss for the slow drainage phase was calculated by subtracting the soil water content when monitoring ended (point E in figure 2) from that when rapid drainage ceased. Table 1 shows the amount of water lost during every phase of each experiment.

Figure 2 Typical curve produced by the irrigation experiments.

Table 1 Water loss during the three phases of soil water content evolution (units in mm)

The average percentage of water loss to total water depth for the wetting phase was very similar for plots 2, 3 and 4, being 28.0, 31.3, and 26.6 % respectively, while for plot 5 it was 13.1 %. Water loss during the wetting phase represented almost 50 % of the rainfall depth in plots 2, 3, and 4; the average values for all runs were 42.6, 43.3, 39.6, and 19.2 % for plots 2, 3, 4, and 5 respectively.

4. Discussion

4.1 Runoff

Overland flow occurred under both natural rainfall and irrigation on plot 1. However, as pointed out by Bergkamp et al. (1996), overland flow may be generated in semiarid areas in a small scale, but it is absent or negligible at larger scales due to the rapid infiltration of water. Factors responsible for this rapid infiltration of water into the bedrock are the numerous gaps among strata, their inclination, limestone karstification, and the action of roots and rock fragments, which obstructed downslope water movement and promoted its infiltration through cracks. Therefore, the dominant processes on the south-facing slope at the small scale are overland flow and infiltration, and at the large scale the most important process is infiltration.

On the north-facing slope, the forested plots did not generate any overland flow. Ponding did not occur at all on the surface even after the continuous application of water and even at the highest rainfall intensities and under high antecedent water content. The soil is extremely porous and has high hydraulic conductivity, allowing rapid infiltration, storage and vertical transmission of surface water inputs. An additional factor is the existence of a large number of cracks and holes created by the activity of fauna and decayed plant roots.

4.2 Soil water dynamics in the forested plots

During the rapid wetting phase water started to infiltrate through the O and the A horizons. During this phase macropore and matrix flow act simultaneously with macropore flow being more significant because of the large irrigation intensities and the very macroporous soil (see Table 2).

From point B to C there is a marked reduction in the rate of wetting. Point B seems to represent the moment when water reaches the bedrock and deep seepage starts to be a significant water flux. During the rapid drainage phase macropore flow is the most important process demonstrated by the steep recession limb of the curve once water irrigation has ended. Irrigation ended at 45 minutes and the soil water content was 0.577 m³ m^-3; 13 minutes later water content had decreased significantly to 0.489 m³ m^-3. Therefore, by employing the drainage approach reported by German (1981) for soil samples and by Navar et al. (1995) for soil blocks, the volume of macropores is the volume of rapid drainage from point C to point D; 0.577 - 0.489 = 0.088 m³.m^-3. This means that 15 % of the A horizon is macropore space. This figure is larger than figures recorded for mineral soils by Germann (1981), Navar et al. (1995), or by direct measurements of macropore space (Bouma, 1981). Table 2 shows the values of macropore space calculated for all runs.

Table 2 Macropore space of the “O” and the “A” horizons calculated for all runs.

The average values of macropore space are 11.9, 15.8, 15.8, and 13.5 % for plots 2, 3, 4, and 5 respectively. The values of macropore space for every plot were subtracted from the total porosity in order to calculate micropore space; these values of micropore space, which determine the water retention capacity of soil, are 50.2, 52.6, 48.9, and 55.9 % for plots 2, 3, 4, and 5 respectively. This could explain why, in spite of the rapid drainage of large amounts of water through macropores, much water remains retained within the micropore system of soil. The two contrasting properties are critical in determining both the short- and long-term water balance of these slopes. Finally, from point D to point E water drains more slowly suggesting the predominance of matrix or micropore flow.

4.3 Water pathways

In plot 1 the main processes are rainfall interception, overland flow and infiltration into the bedrock. Interception represents 13 % of precipitation. Overland flow, at the plot scale, ranged from 27 % to 34 % of the total rainfall for storms with a 1-year return period. Neglecting evapotranspiration, infiltration into the bedrock represents between 53 % to 60 % of the gross precipitation for storms with a return period of 1 year. However, if we consider that overland flow eventually infiltrates into the bedrock, then infiltration represents 87 % of the gross precipitation.

For the forested sites rainfall interception is very similar for all plots, averaging 14 % of gross precipitation. On all forested plots, overland flow was absent and subsurface flow was almost totally absent. The most important processes in the forested sites are infiltration, macropore flow and deep seepage. Soil water storage is also an important component, especially in these sites, where the storage capacity of soil is large. The above mentioned water pathways in the forested sites highlight the influence of trees on water dynamics.

5. Conclusions

The study area has some of the characteristics of both temperate and semiarid regions. It does not, however, behave hydrologically like either type of regime. Given the characteristics of soil and bedrock, it might be expected that either surface flow as Horton flow, or subsurface stormflow would be likely to occur. In fact, in contrast with many semi-arid regions Horton flow does not occur, and in contrast with many temperate regions, subsurface stormflow is minimal.

Vertical gradients of potential dominate on these slopes, and the resulting processes of infiltration, percolation through macropores, and deep seepage, are the most important water fluxes on them. The particular combination of bedrock, soil, vegetation, and climate characteristics, as well as the transitional nature of the area are the reasons for the unexpected characteristic hydrologic behaviour of the study slopes. The observations also support the idea that there is a continuum of mechanisms of runoff production possible not only at the slope or watershed scale, but at the regional scale, depending on the particular combination of controlling variables at any site. In other words, the runoff generation mechanisms which are operative in any given context are not determined by the general conditions attributed to a region, but to quite site-specific controls.

6. Literature cited

Bergcamp, G., L. H. Cammeraat, and J. Martinez-Fernandez. 1996. Water movement and vegetation patterns on shrubland and an abandoned field in two desertification-threatened areas in Spain. Earth Surface Processes and Landforms, 21: 1073 - 1090.

Bouma, J. 1981. Comment on ‘Micro-, meso- and macroporosity of soil’. Soil Sci. Soc. Am. J., 45: 1244 - 1245.

Germann, P. 1981. Unterschungen über den Boden wasserhaushalt inn Einzugsgebiet Rietholzbach, Mitt. Versuchanst. Wasserbau Hydrol. Glaziologie ETH Zurich, 51, 133 pp.

Marroquin de la F., J.S. 1987. Algunos aspectos ecologicos del Bosque-Escuela. Demostracion del Bosque-Escuela/Guia Tecnica, Facultad de Ciencias Forestales, U.A.N.L., Linares N.L., Mexico.

Navar, J., D. Turton, and E. Miller. 1995. Estimating macropore and matrix flow using the hydrograph separation procedure in an experimental forest plot. Hydrological Processes, 39: 743 - 753.

Woerner, M. 1990. Los suelos del bosque escuela de la UANL en la Sierra Madre Oriental, Iturbide, N.L. Reporte Cientifico No. 2. Facultad de Ciencias Forestales, Linares, N.L., Mexico.