Rapid Rate of Travel Evaluation

Rapid Rate of Travel Evaluation of Connection between Nitrate in Root Zone and Groundwater as Affected by Crop and Soil Management

This MPEP project is a demonstration project to allow growers to assess the applicability of rigorously controlled irrigation and fertigation, which have been shown to significantly reduce the rate of nitrate leaching to groundwater in high-frequency low-rate (HFLR, drip or microspray) systems.

Rapid Rate of Travel Evaluation of Connection between Nitrate in Root Zone and Groundwater as Affected by Crop and Soil Management (Rapid Rate of Travel Evaluation) is one of several efforts to expand our knowledge base about this connection, which is fundamental to the Irrigated Lands Regulatory Program. Related efforts include a highly monitored experimental site near Modesto (UC collaborators David Smart, Thomas Harter, and Patrick Brown are investigators), and gathering and assessment of existing shallow groundwater data from other sites (working with Thomas Harter and others).

This project will demonstrate that information about the fate of water and nitrogen (N) in the root zone, as determined by crop and soil measurements, can be used to infer the amount of nitrate moving into groundwater, and that even in a well-managed, HFLR irrigated orchard, environmental performance can be significantly improved through integration of soil moisture monitoring with automated operation of irrigation and fertigation. This project also will demonstrate strategies that prolong N residence time in and uptake from the root zone, even as other salts continue to move outward, to avoid damaging levels of salinity.


Budget and Funding

The overall budget for this project is $354,000 over three years. Currently funded by a Conservation Innovation grant from NRCS. Additional funding will come from the MPEP Committee. Project collaborators are also contributing some of their costs, and the grower on the study site will apply to Environmental Quality Incentives Program (EQIP) and/or other grant funds for up to $70,000 for irrigation, fertigation, and soil moisture monitoring equipment to facilitate shifts in irrigation and fertigation operations to boost production while diminishing leaching of N fertilizer.


The goal of this study is to equip growers with information necessary to evaluate technology-based irrigation and fertigation management that can increase nitrogen use efficiency (NUE) and water use efficiency (WUE) in existing or new HFLR irrigation systems. This goal will be achieved by demonstrating production and water quality benefits of, and increasing access to funding for, soil moisture management (SMM) technologies and irrigation and fertigation system control.


The Rapid Rate of Travel Evaluation Project has the following objectives:

  1. For shallow- and deep-rooted perennial crops, quantify the yield, crop quality, WUE, and NUE benefits of converting from a non-automated irrigation system (operated weekly) to widely available, replicable systems that provide more frequent and precisely timed irrigation and fertigation through automation and SMM feedback.
  2. Evaluate the relationship between these management changes and reductions in the rate of nitrate transiting to groundwater.
  3. Work with growers, commodities groups, and NRCS to develop an initiative that would facilitate cost-share funding of these types of system upgrades, encouraging and enabling their broader adoption. While not strictly a part of this study, this activity is essential to facilitate replication of the work at a scale that will have the desired impact.


This project seeks to demonstrate and replicate the effects of practices that have already been shown to increase the efficiency of applied N fertilizers, with the express goal of expanding the rate of adoption of these and related practices over a very wide area in which HFLR irrigation systems are, or soon will be installed. Automated systems facilitate more frequent irrigation, better spatial segregation (zonation) of application, and better control of fertigation operations, all of which reduce movement of nitrate from the low-salinity, wetted zone surrounding the point of application. Studies have demonstrated that spoon-feeding of N (applying continuously or in smaller, more frequent applications, timed at the end of irrigation sets), tends to reduce the risk that applied N will be leached by subsequent irrigation (Smart et al., 2015; Hanson et al, 2006).

The field study will be conducted on a medium-sized farm on the Kings River fan near Fresno, California. The site includes 29 acres of oranges (a relatively shallow-rooted tree crop) and 18 acres of almonds (a deeper rooting tree crop). On the upper Kings River fan, highly permeable soils overlie groundwater at shallow depth. These conditions increase the likelihood that nitrate not taken up by the crop will exit the root zone and eventually move toward the underlying groundwater. A schematic of this process, and how it can be influenced by management, is shown in Figure 1.

Figure 1. Conceptual cross section of microspray wetted soil volumes, showing movement of N (green), water (blue), and salt (black), as well as a zone of N and salt accumulation at the margins of the wetted volume. Timely irrigation and fertigation enabled by SMM and system automation minimize untimely and excessive leaching toward margins of the wetted root zone, thereby conserving water and maximizing NUE. This also minimizes the amount of N leached to groundwater.

Preliminary site investigations suggest that under the current the management regime, soil in the oranges’ root zone does not have sufficient available water capacity (AWC) to avoid stress between weekly irrigation events during mid-summer. Resulting water stress and overwhelmed AWC reduce NUE and yield. The almonds, on the other hand, have more uniform growth and production, and have accumulated salts around the margins of the wetted soil volume, suggesting frugal but sufficient irrigation, and adequate AWC (in the greater rooting depth). Unfortunately, nitrate has accumulated at these margins along with other salts, in a zone that is now too salty for almond roots to use. The project approach will be to compare management practices, yield, fruit size and quality, plant N status, salinity distribution, and water and N balances. The site will be operated manually during the first two years, then switched to an automated system in which irrigation events are triggered by soil moisture conditions for the last two years.

During the first two years of the project, we will monitor irrigation and fertigation system operation, yield, quality, plant-tissue N levels, salinity distribution, and soil nitrate under the existing manual management regime. These baseline results will be supplemented by grower-maintained data (irrigation dates, run times, CIMIS calculations, soil moisture readings, fertilization amounts and dates, yield quantities and quality) from previous years. The management regime will shift when the system is automated during winter of 2018/19. N-balance monitoring will include N-application rate, timing, form, and placement, along with soil nitrate (ammonium is minimal) and electrical conductivity. This will be supplemented by two electromagnetic induction (EMI) surveys (2017 and 2020) that will indicate where and in what concentration salts accumulate (indicating the margin of the frequently wetted soil volume, and indirectly, the leaching fraction) under each regime.

Tasks and Schedule

The table below presents the project tasks by year.

References Cited and Related Articles

Baram, S, V. Couvreur, T. Harter, M. Read, P.H. Brown, J.W. Hopmans, D.R. Smart. 2016. Agricultural Water Management 172:83–95.

Gardenas, A.I., J.W. Hopmans, B.R. Hanson, and J. Simunek. 2005. Two-dimensional Modeling of Nitrate Leaching for Various Fertigation Scenarios under Micro-Irrigation. Agricultural Water Management 74: 219–242

Hanson, B.R., J. Simunek, and J.W. Hopmans. 2006. Evaluation of Urea–Ammonium–Nitrate Fertigation with Drip Irrigation using Numerical Modeling. Agricultural water management 86: 102 -113.

Smart, D.R., T. Harter, J. Hopmans, P. Brown, S. Baram, and R. Davis. 2015. Optimizing the Use of Groundwater Nitrogen for Nut Crops. Fertilizer. California Department of Food and Agriculture Fertilizer Research and Education Program Proceedings, pp. 35-38.

Zotarelli, L., M.D. Dukes, and R. Muñoz-Carpena. 2009. Soil Water Distribution and Nitrate Leaching of Drip Irrigation Controlled by Soil Moisture Sensors. In Estudios en la Zona no Saturada del Suelo. Vol IX, O. Silva et al., Barcelona.

For more Information

For more information about the Rapid Rate of Travel Evaluation Project, contact the project leaders:

Mr. Casey Creamer
Coordinator, SSJV MPEP Committee
Kings River Watershed Coalition Authority

Dr. John Dickey
Technical Program Manager

Dr. Ken Cassman
Senior Agronomic & Soils Advisor