Border agents apprehended 132,887 people trying to cross the Southwest border in May, a 13-year high, and many of the migrants were asylum seekers. The increased flow across the border has overwhelmed our asylum system. It has become so difficult to get an asylum application processed in the United States that some migrants are giving up and applying for asylum in Mexico.

But the Southwest border is only 1,954 miles long, less than two percent of the 101,471-mile border around the United States (6,000 miles of land borders and 95,471 miles of coastline borders).

And DHS seems to have concentrated its resources on the Southwest border. In fiscal 2017, there were 461,540 apprehensions, but only 3,027 of them were at the northern border and only 3,588 at coastal borders.

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Congress, however, apparently wants DHS to patrol the entire border — all 101,471 miles of it.

Section 1092 in the Fiscal 2017 National Defense Authorization Act (NDAA) requires the DHS Secretary to provide congress with annual metrics reports on securing the border between all of the ports of entry along the perimeter of the United States.

The report must include the percentage of the illegal crossers who were apprehended, how many undetected illegal crossings occurred, how many migrants succeeded in making an illegal crossing but went back to Mexico right away, and how many were detected but able to get away without being apprehended.

The NDAA requires the U.S. Government Accountability Office (GAO) to analyze the suitability and statistical validity of the data provided in the report.

The Secretary submitted the fiscal 2018 Border Security Metrics Report on Feb. 26, 2019, and GAO released its evaluation in March. Among other things, GAO expressed concern about DHS's failure to systematically review the reliability of the data it used to make its determinations and to comprehensively identify and communicate limitations.

I suspect that such a review would just have revealed the futility of the project — here's why.

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Apprehension rate

To determine the apprehension rate between ports of entry, DHS must divide the number of apprehensions by the number of unlawful entry attempts.

For example, if the Border Patrol apprehends 100,000 illegal crossers next month, and there are 100,000 unlawful entry attempts that month, the apprehension rate would be 100 percent. Every alien who attempted an illegal entry would have been caught. But if there are 200,000 unlawful entry attempts and 100,000 apprehensions, the apprehension rate would be 50 percent.

DHS has developed two systems for making this calculation, a model-based apprehension rate system and an observational apprehension rate system. See page 7 of the report.

The apprehension rate produced by the model-based system for fiscal 2017 was 65.4 percent, and the observational-based system produced a rate of 74.5 percent.

I don't think these systems can produce reliable information. If you don't know how many illegal entries were attempted, you can't calculate an apprehension rate, and creating sophisticated-sounding metrics systems doesn't change that fact.

DHS could do this on the land borders by installing surveillance equipment, known as a "virtual wall," but an attempt to install such a system in 2006 was a complete fiasco. When the project was terminated in 2011, only 53 miles of the system had been installed — at a cost of one billion dollars.

Detected unlawful entries

DHS does this calculation by adding the number of "turn backs" and "got aways" to the number of apprehensions.

"Turn backs" are migrants who, after making an illegal entry, respond to U.S. enforcement efforts by returning promptly to the country from which they entered.

"Got aways" are migrants who are observed making an illegal entry but are not caught.

According to the report, the number of detected unlawful entries fell from 2 million in fiscal 2006 to roughly 500,000 in fiscal 2017, a 75 percent decrease.

Undetected unlawful entries

This refers to the number of migrants who succeed in making an illegal entry without being detected. DHS developed a system for making this calculation, but that doesn't change the fact that they are just guessing.

According to their system, undetected illegal entries fell from more than a million in fiscal 2006 to fewer than 57,000 in fiscal 2017, a 95 percent decrease.

Turn backs

According to DHS, the primary limitation to detecting turn backs is that it requires subjective observations from thousands of individual agents.

The report concludes that there were 91,998 turn backs in fiscal 2017.

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Got aways

The report only mentions got aways that occurred on the Southwest border.

It states that got aways declined from 615,000 in fiscal 2006 to 104,000 in fiscal 2017, an 83 percent decrease.

Although DHS should provide the data needed to determine the effectiveness of border security measures, it won't be able to acquire it without costly surveillance systems that may not be feasible to install with current technology.

Until then, DHS estimates will just be educated guesses.

Nolan Rappaport was detailed to the House Judiciary Committee as an executive branch immigration law expert for three years. He subsequently served as an immigration counsel for the Subcommittee on Immigration, Border Security and Claims for four years. Prior to working on the Judiciary Committee, he wrote decisions for the Board of Immigration Appeals for 20 years. Follow him on Twitter @NolanR1