Basin Description

As described in the reports of the PROCUENCA-SAN JUAN project, the SJRB is located between 10º and 12º 30’ north latitude and between 83º 30’ and 86º 15’ west longitude. The SJRB encompasses southeast Nicaragua and northeast Costa Rica. Its boundaries are defined by the basin of Lake Nicaragua and its tributaries, together with the San Juan River and its tributaries, and their allied coastal zone (Map 2). The basin includes 37 municipalities in Nicaragua and 7 cantons in Costa Rica (Table 1).

Table 1: Municipalities (Nicaragua) and Cantons (Costa Rica) in the SJRB.

Most of the basin is below 500 meters above sea level and rises from the coastal Caribbean flood plains of Indio Maiz (in Nicaragua) and Tortuguero (in Costa Rica) to the 1500-3000 meter highlands of northern Costa Rica and to just over 1,600 meters above sea level in Nicaragua. Rainfall is greatly influenced by this topography and reaches between 4,000 mm to 6,000 mm over nine to eleven months in the more humid sections, and between 1,000 mm and 2,000 mm in the drier zones around Lake Nicaragua that have a dry season of about seven months (Map 3, Table 2).

Table 2: Monthly and annual precipitation in the SJRB (mm).

This precipitation has built up abundant and high-quality groundwater which, together with Lake Nicaragua, amounts to the most significant freshwater reserve in Central America. The average temperatures in this region are between 22 °C and 24 °C, but in the tropical forests, monthly averages are between 23°C and 28°C. In the highest regions of the river basin along its southern limit, temperatures drop to 9°C in the rainy season and 11°C in the dry season. Relative humidity is also high; in the tropical rainforest, it attains more than 90%, while in the low northern savanna it is 62%. Evaporation rates, of course, vary with temperature and relative humidity. In the Nicaraguan area of the basin, evaporation rates have been recorded from 2,376 mm in Altagracia (62 meters above sea level) to 1,748 mm in Boaco (360 meters above sea level).

Lake Nicaragua's only outlet is the San Juan River. Initially the river is entirely within Nicaragua, but approximately five kilometers downstream from El Castillo, its right (southern) margin becomes the international border between the two countries. The river strikes a course towards the southeast and, 205 km from its start, it separates into two branches that empty into the Caribbean approximately 20 km apart.

As can be expected, this variation in elevation, topography, and distance from the sea has created at least 14 different life zones that vary from very dry tropical forest to montane rainforest (Map 4).

Climate Variability in the SJRB: Data and other information from the National Emergency Commission (CNE) of the Meteorological Institute (IMN) of Costa Rica, and the Nicaraguan Institute of Territory Studies (INETER) that relate to threats from climate variability were used to describe climate and climate variability in the SJRB. Information from these units covers nearly 100 years and includes nearly 25 years of data on ENSO, the El Niño Southern Oscillation. The influence of ENSO on precipitation and drought is a major item of discussion concerning climate variability, although it should be noted that periods of drought occur without the presence of El Niño, as was the case in 2001. Flooding, one of the most frequent causes of disasters in the basin, happens almost annually in the lower-lying areas. Despite this, it is common to find crops, human settlements of several different social classes, and other constructions in flood prone areas. Flooding, likewise, does not always depend on an ENSO episode and can be caused by persistent rainfall over the same area, heavy rainfall even if for a short period, obstruction of its flow caused by landslides, or the breaching of a dam.

Hurricanes and tropical storms have passed directly over the basin (Map 5) and the influence of storms four to five hundred kilometers away can have a major influence in it. Despite this, only four of the 37 administrative units have a high level of hurricane risk relative to the rest of the units in the basin (Table 3). Indeed, municipal offices in the basin seldom report damage caused directly by hurricanes, although they do say that flooding from persistent rainfall, tropical depressions, and runoff from deforested areas are problems for their communities. Like much of western Central America, however, much of the SJRB regularly suffers from drought and 31 of its 37 municipalities and cantons have a high level of drought risk (Table 3).

Climate variability is different from “climate” which, within a given geographical area, is the averages of temperature, wind direction and speed, atmospheric pressure, humidity and a number of other meteorological parameters, calculated over a sufficiently long period so as to be called "normal." On the other hand, “climate variability” is dependent on extreme atmospheric conditions that far exceed the normal. The phenomena that produce these extremes are highly organized cold fronts, dry stationary cells, hurricanes, tropical disturbances, and cells with disproportionate humidity. Paradoxically, extreme meteorological events can include both excessive rainfall and extended drought. As in many parts of the world, such phenomena in this region are thought to be associated with ENSO—the El Niño Southern Oscillation.

Table 3: Levels of risk for hurricanes and drought in the cantons and municipalities of the SJRB

ENSO is a cyclical phenomenon initiated with changes in temperatures in the tropical Pacific Ocean (Map 6).⁴ The two phases of the cycle (El Niño, the warm phase, and La Niña, the cold phase) influence air pressures, precipitation totals, and temperatures worldwide, which can be above or below normal depending on where they are being measured. Significantly, El Niño episodes cause more precipitation across the Pacific to about 86º west latitude, and less precipitation from there to the east; La Niña episodes do just the opposite (Figure 1). Consequently, since Costa Rica and Nicaragua are at the precise latitude where the effects of El Niño change, ENSO impacts vary depending on where one is in the two countries. Though well-defined effects do occur when the intensity of El Niño is strong—for example, in 1982 and 1997, when rainfall totals in the area were far below normal. The 1997 occurrence was probably the strongest of the twentieth century, and the Pacific Slope and Central Valley of Costa Rica experienced a large deficit of precipitation, while the Atlantic slope had a 40% increase above normal. Interestingly, however, during 2001 all of Central America suffered conditions of drought without the presence of an ENSO event.

Each phase has an average period of recurrence of about four years although the historical record shows variations ranging from two to seven years. Generally, each El Niño episode lasts between nine and twelve months beginning June to August. Most peak during December to April and decay during May to July of the next year. However, some episodes may last two years or longer. Recent research also suggests that ENSO can influence the formation of hurricanes where a strong El Niño can inhibit their formation, while a strong La Niña episode can provide relatively more favorable conditions for hurricane formation in the Atlantic.

For example, the regional drought of 1997-98 in Central America was an El Niño event, while the two most recent important hurricanes to hit Central America (Cesar in 1996 and Mitch in 1998) occurred when La Niña was active. Hurricane Cesar was one of the more destructive in Costa Rican history and Hurricane Mitch is considered the most destructive in all of Central America, causing thousands of deaths and injuries, and billions of dollars in damage to housing, infrastructure, agriculture, ranching, and fishing—all of which continue to inhibit socio-economic development throughout the region. Although the brunt of this hurricane—one of the worst to ever strike Central America, was felt more severely far to the north of the SJRB, its effects were considerable within the basin. To illustrate, Figure 2 shows the increase in rainfall at several stations in Nicaragua during the month of October 1998. Of the 14 stations listed, Masatepe, Nandaime, Rivas, Ocotal, Muy Muy, San Carlos and Juigalpa are all within or very near the basin.

There is no doubt that individuals and communities in the basin are inadequately protected from climate variability—and they suffer accordingly. Poverty, which acts to magnify the negative effects of climate extremes, does not often allow the extra resources required to prevent or adequately mitigate problems brought on by climate variability. However, old and new communities remain in the basin and, however insufficient, coping mechanisms do evidently exist and appear to have some degree of success.

Figure 2:	Precipitation at several station in Nicaragua for October 1998 compared with historic averages

Hydrology in the SJRB: The drainage network (Map 7) shows that the rivers of the northern sector of the basin are short in length, generally oriented north to south, and eventually find their way to Lake Nicaragua. Most of the rivers in the southern sector of the basin originate in Costa Rica, in the Cordillera de Guanacaste to the west, and at elevations of up to 3,000+ meters in the Cordillera de Tilarán to the east. The high levels of precipitation along the northern flank of the Tilarán range contribute approximately 85% of the San Juan River’s total volume.

Estimated flows of the river are 475 m³/s at San Carlos, at the outlet from Lake Nicaragua. They increase to 833 m³/s just before the confluence with the Sarapiquí River,

and 1,308 m³/s at its outlet to the Caribbean. Twenty-six percent of the river's total flow at its mouth originates from Lake Nicaragua; 6.5% from inflow between San Carlos and El Castillo; and 67.5% between El Castillo and Sarapiquí.

Groundwater in the SJRB is abundant and of high quality, except on the Caribbean coastal plain, where it is salty. Aquifers with high-quality water have been found between 45 and 105 meters at the southern and western boundaries of the basin, and at 8 to 40 meters on the interfluvial plains on both sides of the Rio San Carlos, where they supply potable water to the local populations.

Riverine sediments originate in the upper and middle parts of the basin’s watersheds and are the result of strong rains, the fragility of many volcanic soils, deforestation, road construction, and agricultural and livestock development. Lake Nicaragua is the depository for sediments carried by its tributaries. It also receives laminar runoff and subsurface drainage carrying dissolved, or suspended pesticides and fertilizers. Little is known of the volume of sediment arriving at Lake Nicaragua. At the Terrón station on the San Carlos River in Costa Rica, the contribution of sediment was calculated at 817 tons/Km²/year; at Peñas Blancas, 700 tons/km²/year; at Punta Viejo and Veracruz on the Sarapiquí River, it was calculated at 216 tons/km²/year. At the Guatuso station on the Frío River, a contribution of 298-tons/km² year was calculated. In Nicaragua, agricultural activity and deforestation have caused erosion problems and, as a result, floods and the diversion of watercourses. Periodic samples of suspended sediments have not been taken at key stations; however sedimentation can be verified in that some riverbeds have been filled in with the consequent problems of overflow, flooding, and formation of new sand and mud bars.

Agricultural land covers 60% of the territory. In Nicaragua, the basin produces 26% of the national bean crop, almost 21 % of the sorghum, and slightly more than 16% of the corn and sugar cane. In the Costa Rican sector, it produces 54% of the nation’s beans, 11% of its sugar cane, 6% of the corn and banana production, and 90% of tubers and root production. The agricultural frontier in Nicaragua is still expanding—often on land of low productive capacity and on lands within protected areas (the agricultural frontier is expanding eastward to threaten the Indio-Maíz Biological Reserve). In Costa Rica, the productive change from forest to agriculture has already occurred and pressure is also directed at the colonization of protected areas.

The causes of this process are found in a combination of factors linked to subsistence of agriculture, clearing forests for rangeland, inefficient use of natural resources, and newly arrived migrants looking for land to settle. Lumber extraction has often been an additional factor, providing road access and people to the colonization process. Production technologies, which are in many cases inappropriate, and the intensive use of agrochemicals on certain crops, have negatively affected water quality.

Agriculture in the SJRB has clear asymmetries with respect to the number of producers and available land areas. Commercial producers represent 11% of the total number of farms, yet, they occupy 55% of the agricultural land while small and subsistence producers represent 89% of the farms on but 45% of the total agricultural area.

Ranching plays a fundamental role in both sectors of the basin, where nearly 20% of the national herd for each country is found. The herds are different, however, in that those in Nicaragua are generally dual purpose, meat, and milk, while in Costa Rica the production is specialized. Genetic improvement of the herds is a priority, as are sanitation and range and pasture management.

Associated industrial activities are located in the basin, where 70 sawmills operate (10 in Nicaragua and 60 in Costa Rica). On the Costa Rican side, four banana companies are located in Pococí; one citrus processing plant in Los Chiles; three sugar mills and milk cooperatives in Quesada City and San Carlos; and numerous coffee processing plants scattered in the coffee growing areas. On the Nicaraguan sector, agroindustrial activities are the sugar mills in the Department of Rivas, one tomato processor in the municipality of Nandaime, one palm oil extraction plant in El Castillo, and several coffee processing plants.

Socio-Economic Conditions: An estimated 1,070,000 people live in the basin, 780,000 (70%) in Nicaragua and nearly 290,000 (25%) in Costa Rica. Over 40% of the population of the Nicaraguan portion lives in but four of the 37 municipalities and, within these, in just three cities (Masaya, Granada, and Juigalpa). Fifty-five percent of the population is rural and, although scattered unevenly throughout the basin (in part, because of the number and size of the basin’s parks and reserves), the population density in Nicaragua is 46/km². On the Costa Rican side, 85% of the population is rural and the population density, at 22/km², is less than half that of Nicaragua. Most are generally very poor and lack access to safe drinking water and adequate sanitation. Furthermore, all economic and quality-of-life indicators for these populations are significantly below the national averages for both countries. Because of an imbalance in employment and income-generating opportunities between the two countries, migration into the Costa Rican portion of the basin during times of stress exceeds the capacity of existing local institutions to meet the needs this creates. Table 4 is instructive on this issue:

Table 4: Comparative land use figures for Costa Rica and Nicaragua in the SJRB

*Includes lakes and conservation and protection areas.

The basin's population has traditionally maintained strong social and commercial ties across the border. It is common to find family relationships among them. However, as in many parts of the region, population growth over the last 20 years on both sides of the border brings in “outsiders” and change—not all of it positive. All quality-of-life indicators are lower in both parts of the basin than their respective national averages. The cantons of Upala, Los Chiles, Guatusos, and La Cruz, in Costa Rica, have significantly lower social development indicators in housing, education, electrification, sewage, communications, and health—the last even deteriorating in recent years. While the Nicaraguan portion of the basin has 18.4% of the national population, it also has 36% of the poor population and 43.2% of the nation's indigents.

Costa Rican Sector: Costa Rica’s “Social Development Index” is based on a number of variables including education infrastructure, access to special education programs, infant mortality, deaths between the ages of zero and five relative to the general death rate, height of the population entering the first grade, average monthly use of electricity and births to unwed mothers. The index can vary between 0 and 100 with the higher numbers indicating better social conditions. For the cantons in the SJRB, index figures average 28.7 with a high of 45.5 and a low of 8.9. Four of the cantons fell below 30 and three were above 40 (Table 5).

Table 5: Social Development Index for Costa Rican Sector of the SJRB.

Because of job scarcity in Nicaragua, laborers tend to migrate from Nicaragua to Costa Rica, where more work opportunities exist. This flow of migrants is higher during the sugar cane harvest periods and, since many of these become permanent immigrants, an agreement between both countries regulates the phenomenon.

Still, the population in the Costa Rican sector has doubled in the last 20 years. Now, the rural population represents 85% of the total and the economically active population is 44%. The primary sector, where payment tends to be made according to productivity, employs nearly 73% of these. In the last decades, except for agriculture, real salaries have tended to decrease and they are seldom sufficient to satisfy basic needs.

Population in rural areas is scattered and the majority lack basic services and sanitation. Ninety-nine percent of the homes in population centers have public water supply. By contrast, only 62% of the homes in rural areas have water supply service. Water in the SJRB is supplied through storage tanks and a distribution system, springs, drilled and hand dug wells, rain catchments (cisterns), or directly from a river. Only the first source is of guaranteed quality; the remaining sources are risky at best.

Sixty-one percent of the population uses septic tanks and the remainder use latrines and cesspools. Any of these systems is susceptible to overflow during flooding with the rainy season (especially in the low-lying areas), causing contamination of wells, streams and rivers; resulting in intestinal and other parasitic illnesses; and propagating disease vectors that can cause epidemics of dengue fever and malaria.

Solid waste collection services are available to just 32% of the urban population and 0% of the rural population. Consequently, almost all solid waste eventually ends up in the river.

Basic education is limited—there are approximately 640 primary and 40 intermediate level education centers in the entire Costa Rican portion of the basin.For this reason, and due to the need to begin work at an early age, the literacy rate is 84.6%, high for many parts of the world, but low for Costa Rica.

Health coverage is also deficient with just 24 confinement centers (clinics and hospitals) for a population of nearly 290,000 inhabitants. The area has additional rural health posts, children centers that offer full health attention, and health education and nutrition centers providing outpatient services.

Nicaraguan Sector: A significant population increase has also occurred in the Nicaraguan sector during the last 30 years, from 300,000 inhabitants in 1971 to an estimated 780,000 currently. Unemployment and underemployment rates in the basin are higher than in the rest of the nation. Nearly one-half of the population is under 15 years of age.

All social indicators show significantly low values (Table 6). Approximately 93% of the urban population and 39% of the rural population are provided with potable water service—principally from groundwater sources because of the population centers east of Lake Nicaragua. Water is provided through urban and rural piped distribution systems and manually pumped wells. A reduction in service can occur during intensely dry periods.

Table 6: Selected social indicators from the Nicaraguan Sector of the SJRB.

Twenty-two percent of the urban population is served by sewage disposal systems. Much of the rural population uses latrines—between 38 and 79% depending on the area, although the San Juan River Department has a coverage rate of just 17%. Solid waste is collected only in urban areas, but is limited with respect to equipment and efficient operation.

The average literacy rate is approximately 80%, and in the rural areas it reaches only 55%. Health services are deficient; the average ratio is 710 inhabitants/bed and the San Juan River Department, at 1,362 inhabitants/bed is far below the remainder of the basin. The average number of inhabitants per home is six.

Political-Administrative: Map 8 shows the political/administrative divisions of the SJRB. In Costa Rica, the political-administrative division is by provinces and cantons—seven of which are entirely within the SJRB (La Cruz, Upala, Guatuso, Los Chiles, San Carlos, Sarapiquí, and Pococí). A few others are only partially within the basin and are omitted from the project. In Nicaragua, departments and municipalities make up the political/administrative units. One of these, the San Juan River Department, lies entirely within the basin, as do parts of five others (Masaya, Granada, Boaco, Chontales, and Rivas). Within the project area, these departments are further divided into 37 municipalities. Most of the available information for describing the basin comes from data captured at the municipal, canton, province, department, and national levels. Very little of the information required in this characterization is available from data gathered within physical or biographical boundaries.

4 - Map is from the NWS MLB Digest for February-April 2002 found at www.srh.noaa.gov/mlb/digest/issue20/ ninonote.html