Evaluation of Hydrological Processes and Environmental Impacts of Free and Controlled Subsurface Drainage
Statistical analyses, including paired watershed approach and paired t-test, indicated that controlled drainage had a statistically significant effect (p-value <0.01) on the rate of water table fall and reduced the water table recession rate by 29% to 62%. The slower recession rate caused by controlled drainage can have negative impacts on crop growth and trafficability by causing the water table to remain at a detrimental level for longer. This finding can be used by farmers and other decision-makers to improve the management of controlled drainage systems by actively managing the system during storm events.
A method was developed to estimate drain flow during missing periods using the Hooghoudt equation and continuous water table observations. Estimated drain flow was combined with nutrient concentrations to show that controlled drainage decreased annual nitrate loads significantly (p<0.05) by 25% and 39% in two paired plots, while annual soluble reactive phosphorus (SRP) and total phosphorus (TP) loads were not significantly different. These results underscore the potential of controlled drainage to reduce nitrate losses from drained landscapes with the higher level of outlet control during the non-growing season (winter) providing about 70% of annual water quality benefits and the lower level used during the growing season (summer) providing about 30%.
Three different methods including monitored water table depth, a digital photo time series and the DRAINMOD model simulations were used to determine the generation process of surface ponding and runoff and the frequency of incidence. The estimated annual water balance indicated that only 7% of annual precipitation contributed to surface runoff. Results from both simulations and observations indicated that all of the ponding events were generated as a result of saturation excess process rather than infiltration excess.
Overall, nitrate transport through controlled drainage was lower than free drainage, indicating the drainage water quality benefits of controlled drainage, but water table remained at a higher level for longer when drainage was controlled. This can have negative impacts on crop yields, when water table is above a detrimental level, and can also increase the potential of nutrient transport through surface runoff since the saturation excess was the main reason for generating runoff at this field.
Drain flow record:
Using the Hooghoudt equation and measured water table depths, together with linear regression method, a long-term drain flow record for a controlled drainage (CD) experiment at the Davis Purdue Agriculture Center was constructed.
Site description: The Davis Purdue Agricultural Center (DPAC) is a research farm in eastern Indiana. The controlled drainage experimental site is the 0.16 km2 field split into four quadrants, northwest (NW), southwest (SW), northeast (NE), and southeast (SE) with areas of 3.5 ha, 3.5 ha, 3.6 ha, and 3.7 ha. This field has a slope of less than 1%. Soils at the site consist of Blount (somewhat poorly drained), Condit (poorly drained), Pewamo (very poorly drained) and a small portion of Glynwood (moderately well drained) series. The drainage system was installed in 2004. Laterals have an approximate depth of 1 m and spacing of 14m. Drainage in the SE and NW quadrants was controlled during some periods while the SW and NE were allowed to freely drain at all times. A more detailed description of the methods can be found in Saadat et al. (2018).
Nitrate and phosphorus loads:
Daily nitrate-N, soluble reactive phosphorus and total phosphorus loads in subsurface drainage were quantified in an agricultural farm field in eastern Indiana (Davis Purdue Agricultural Center).
Site description: The data was collected from the field W at Davis Purdue Agricultural Center (DPAC) located in eastern Indiana. Field W is relatively flat (slope < 1%), with 0.16 km2 total area, divided into four plots, northwest (NW), southwest (SW), northeast (NE), and southeast (SE) with areas ranging from 3.5 ha to 3.7 ha. The four soil series at the site range from very poorly to somewhat poorly drained, with a small portion of moderately well drained series. The subsurface drainage system was installed in 2004, with 10-cm laterals having an approximate depth of 1 m and spacing of 14 m, resulting in a drainage intensity of 1.1 cm day-1 and drainage coefficient of 1 cm day-1. Drainage in the SE and NW plots was controlled at two different levels during some periods depending on the season, while the SW and NE were allowed to drain freely. This field has been in a corn-soybean rotation since 2011 and in continuous corn before that, and was managed using chisel-plow tillage in the fall and field cultivator tillage in the spring during the study period. Nitrogen (N) and phosphorus (P) fertilizers were applied at different rates prior to and after planting corn. Phosphorus was also applied prior to soybean planting in two of the three soybean years. The rate and timing of fertilizer applications were uniform for all plots and were based on Purdue Extension recommendations. Further details of the site management and data are available in Abendroth et al. (2017). More information about this site and fertilizer application can be found in Saadat et al., 2018.
Sampling strategy and load calculation: Automated water samplers (ISCO) were used to draw samples from the drainage outlet flow of each plot. Samples were collected every hour when flow was present except during winter, and combined into weekly composite samples varying in length from twice a week to biweekly. During the winter, water samples were collected manually to avoid freezing problems, approximately every week whenever flow was present. Samples were kept frozen until analysis and then analyzed on a SEAL Analytical AQ2 auto-analyzer to be tested for nitrate+nitrite-N (referred to nitrate-N), soluble reactive phosphorus (SRP) and total phosphorus (TP) according to US EPA methods.
Daily nitrate-N, SRP and TP concentration values needed for the load calculations were estimated using linear interpolation. After estimating daily concentrations, daily loads were calculated by multiplying the daily drain flow by estimated daily concentrations.
References: Saadat, S., Bowling, L., Frankenberger, J. and Kladivko, E., 2018. Nitrate and phosphorus transport through subsurface drains under free and controlled drainage. Water research, 142: 196-207.
Hourly photos were taken from the Field W located at Davis Purdue Agricultural Center (DPAC) in eastern Indiana using time-lapse cameras. These photos were taken during the daytime from four different plots that are described below.
Site description: Field W is relatively flat (slope < 1%), divided into four plots, northwest (NW), southwest (SW), northeast (NE), and southeast (SE) with areas ranging from 3.5 ha to 3.7 ha. Drainage in the SE and NW plots was controlled during some periods, while the SW and NE were allowed to drain freely.