BACK TO PROJECT LIST

Abiotic Study of the Impact of Flooded Rice Production

on the Tempesque River Watershed

OR

A healthy serving of RICE and GRINGOS

Back to the Classroom    Bibliography     Table 1     Chart 1-nutrients    Chart 2-pH

Abstract:

The marshland and downstream Tempisque River within Palo Verde National Park are fed by waters that are drained from extensive rice production within the watershed. Rice production requires heavy inputs of fertilizer, pesticides and herbicides. Our question considers whether water flowing out of the rice paddies will have greater concentrations of nutrients than water entering from the canals. We tested four sites for indicators of water quality including nitrate, nitrogen/ammonia, phosphate, potassium, total dissolved solids (TDS), chlorine, chloride, temperature, and pH. Test methods included Computer Based Labs (CBL) as well as LaMotte test kits. Sampling sites included the input canal, standing water within the rice paddy and outflow of the rice paddy that leads directly to the Rio Tempisque. Results indicated that most abiotic factors remained unchanged throughout the sample sites with the exception of phosphate, potassium and pH. Farming practices including fertilizer applications and post-harvest chaff burning can account for increased levels of phosphate and potassium found in the rice paddy and output sites. Excessive nutrient loads have been proven to negatively impact water quality.

Introduction:

Costa Rica produces 60 % of the rice produced within Central America, (Swisher and Hatch, 1999). Land within Costa Rica, including the Guanacaste region, is rapidly being converted to agricultural production. Between 1800-1950, 27 % of Costa Rica’s forests disappeared and the larger part of this transformation is directly due to conversions to the cultivation of bananas, coffee and rice (Swisher and Hatch, 1999). The economic value of this food source is significant, and given the importance of rice in feeding the world, and the amount of land that is devoted to rice production, it is essential to fully understand how rice production contributes to ecosystem degradation and global climate change. Our question considers whether water flowing out of the rice paddies will have greater concentrations of nutrients than water entering from the canals.

In Costa Rica, flooded rice is produced in a modern and mechanized fashion (Swisher and Hatch, 1999). Initially, the rice paddies are flooded with ½ inch of water. The soil within the paddy is then churned with either a rototiller or other type of heavy machinery that breaks up the compacted clay so that the rice can take root and grow. After the soil has been loosened, fertilizer is then applied to the fields. The rice seeds are then aerially scattered across the fields by plane. The seeds sink to the bottom and germinate. The water level of the field is then raised as the rice crop begins to grow. Pesticides and herbicides are applied post-emergent to control mildew and "red rice" (a weed that is morphologically similar to the rice crop). The crop’s growth period lasts between 90-115 days. Once the crop is mature, the water is drained from the paddies. When the grain reaches 12 % humidity, combine harvesters are then brought in to harvest the crop. After harvesting, the chaff is burned along with the rest of the rice plant and then the paddy is ready for another crop. Each rice paddy is usually planted two to three times per year with a rotation of stages of production throughout the fields. This allows the equipment to be used most efficiently (Swisher and Hatch, 1999).

Due to the intensive use of these fields for rice production (2-3 crop rotations per year), the soil becomes quickly depleted of nutrients. As a result, continued productivity requires that fertilizers be extensively applied. Fertilizers containing essential plant nutrients nitrogen, phosphorous and potassium in a 10-30-10 ratio are used in rice production. The nutrients not taken up by rice production are left behind and contribute to nutrient enrichments of surrounding aquatic ecosystems. This can have impacts on ecological processes at population and community levels (Vitousek,1994). In addition, net primary productivity may be increased by additions of nitrogen fertilizer, but overall species richness declines (Vitousek,1999).

Rice chaff being burned right after harvesting.

The area studied in this report is called the Barbudal region of the lower Tempisque River within the Guanacaste Province of Costa Rica. This region is considered a tropical dry forest climate and receives rain only 6 months of the year. In order to be able to rotate three rice crops per year in this seasonally dry region of Costa Rica, water must be artificially supplied through irrigation. Irrigation water is supplied by the diversion of surface water from the Arenal reservoir. This water is transported through concrete irrigation canals and diverted from these canals into the rice paddies. Excess water drains from the rice fields into the marshlands located within Palo Verde National Park and then into the Rio Tempisque.

Topographical Map of the Tempesque River Watershed, illustrating the irrigation network.

Methods:

After careful consideration of topography and drainage within this watershed, we chose one of two regions for study. It the first region, the drainage was determined to flow from the rice paddies into the marsh and then the river. It was decided that this site might give ambiguous results in terms of additional inputs not considered by our study. An example is the inputs of fertilizer from cattle grazing.

The region we chose to study gave a more direct relationship between inputs and outputs. The water drainage in this region flowed directly from the rice paddies into the Rio Tempisque. The study group determined that the study sites gave broad representation of fertilizer inputs due to the fact that the rice fields are managed cooperatively (Janzen, 1983) and therefore inputs are more likely to be more uniformly distributed compared to individually owned rice fields. In addition, sampling sites within the region were chosen to represent each stage in the growth cycle of the rice crop.

Site 1: Canal traveling northwest from source (Arenal Reservoir). The sampling point is near a bend in the canal near flow to rice paddy.

Site 2: Flooded and harvested area with recent burning near input of water.

Site 3: Actively growing rice field down hill from harvested field.

Site 4: Output – lowest point of road downstream of outflow in a canal that travels west . Meets up with other outputs through park to river.

The flow of water from site 1 to site 4 traveled in a southwesterly direction across the region.

 Sampling Protocol:

Abiotic water quality indicators were measured quantitatively using standard chemical and computer based lab techniques. Four sample sites were chosen in an area of intensive agriculture characterized by a direct input/output of water, which impacts Palo Verde National Park. A team of five researchers did all testing on site.

Fieldwork was conducted on Wednesday, July 19, 2001 from 10:00 am to 1:05 pm. The ambient temperature was 23 C and the weather was overcast with scattered showers. Samples were collected directly from water sources less than one meter from edges using clean 250 ml collecting bottles.Temperature, pH and conductivity (total dissolved solids/TDS) at all four sites were measures using CBL by Texas Instrument. Probes were inserted directly into the water source and 3 random readings in a 3-meter perimeter were calculated digitally. The pH probe was calibrated using standard buffer solution and distilled water. Wide range pH tests were also conducted using LaMotte Test Kit, which correlated with CBL results.

TDS (conductivity) was calculated using titration methods as per LaMotte Water Pollution Outfit test kit instructions. Aqueous ammonia, nitrogen, phosphate, potassium, and chlorine/chloride tests were calculated using colorimeter techniques as per instructions of LaMotte Test Kits.

Results: We found temperature, Nitrate, Chlorine, TDS, Conductivity, Conductivity to remain consistent from site to site. Phosphate and Potassium levels increased between the intake canal and the rice fields.

Click below to see Table 1	Click on a thumbnail to see enlarged chart
Table 1: Test Results	Chart 1: Nutrients	Chart 2: pH

 

Discussion:

The following 11 abiotic factors were collected. Temperature, TDS, chlorine, chloride, flow rate, pH, nitrate, ammonia, phosphate, potassium. In the results for four of these factors including nitrate, phosphate, potassium, pH, we found fluctuations in levels between sites.

Temperature remained constant through our test plots and was not considered important because we believe that water temperatures equilibrate with ambient temperature on the long journey through warm temperature regions from its source to Guanacaste.

Total dissolved solids, (TDS), in a natural water system are usually composed of sulfate, bicarbonate and chlorides of calcium, magnesium and sodium. For agricultural crops, salinity and conductivity are also to used to describe TDS. Readings of 185 or less is considered low; between 185 and 500 is medium; 500-1500 is high, (LaMotte). We found inconsistencies between our CBL and chemical analysis of TDS. We did discover our distilled water to have a low pH which may have affected our chemical test results. Most of the reading fell in the low range and differences between sites seemed inconclusive.

Chlorine and chloride are tested separately due to their causes and impacts. Chlorine is not present in natural systems and is found only as a result of chlorination of a water supply. Its presence is considered evidence of chlorine treated effluents. Chloride is one of the major anions found in water and sewage. Causes vary, but major changes are considered reasons to suspect pollution. U.S. Public Health Service Drinking Water Standards recommend a maximum content of 250 ppm, (LaMotte). None of these tests resulted in either excess or appreciable change among the sampling sites.

The test for pH indicated that the irrigation water changed from acidic to basic in the course of the flow. This is vitally important since most living things are sensitive to pH. The normal situation in this (geologic) area was normally an acidic input due to the volcanic origins of the soil. It is then neutralized from the limestone (erosion) infusion. Our results matched we expected given the geology of the area.

NPK, (nitrogen, phosphorous & potassium) are the major nutrient elements needed in all living things. They are considered both promoters and limiting factors in growth. These are the component parts of most agricultural fertilizers and dictate growth. Nitrogen is a part of every living cell. Phosphorous is vital in developing plants for root development. In adult plants it is vital for healthy seeds and fruits. Potassium acts as a catalyst, promoting plant metabolism, photosynthesis, efficient use of water and formation of strong roots and stems. Our results show a dramatic increase in both potassium and phosphorous.

Nitrogen did not change and was consistently low. This would indicate a limiting factor. The reasons for this are based in the farming practice. Growth itself accounts for the immediate reduction in nitrogen in the outflow water. Developing plants use nitrogen and incorporate it. After harvesting the rice the remaining plant is burned. This burning liberates the nitrogen that could be used for future plantings, if it were allowed to breakdown and be released into the environment naturally.

Potassium and Phosphorous appearing in greatly magnified levels indicate a lack of absorption and excessive expulsion. This can be traced directly to farming practice. The burning of the remaining biomass after harvesting has an effect on potassium and phosphorous as well as nitrogen. It has been shown that during the burn there is partial loss of volatile elements such as nitrogen and potassium. Further loss can be attributed to the fact that most of the potassium in plants is in wood and stems and therefore in the fuel and ash (Raison, 1979). It is possible that these are further reduced due to the flooding and continuous runoff required for rice production.

The evidence supports our hypothesis that the production of monoculture rice has an impact on water quality within the Tempisque river watershed. The implications of this are that the effects of excessive nutrient loading on a watershed, eutrophication, and further could be the impetus for future study of the source of recent overpopulation of cattails (typha).

Acknowledgements:

We would like to thank don Eliesel Noguera, Park Ranger, Carlos, driver of the bus, doña Viviana Moltiel, National Park cook who so graciously prepared lunch for our outing on the field, and John Cozza, Melanie Phelps, Jim Cole, and Tom Langen from the OTS Staff who provided us with direction, support, and mentoring.